diff --git a/claude_code_log/html/assistant_formatters.py b/claude_code_log/html/assistant_formatters.py
index 3128a50c..02295cf9 100644
--- a/claude_code_log/html/assistant_formatters.py
+++ b/claude_code_log/html/assistant_formatters.py
@@ -108,7 +108,10 @@ def format_image_content(image: ImageContent) -> str:
     src = export_image(image, mode="embedded")
     if src is None:
         return "[Image]"
-    return f'<img src="{src}" alt="image" class="uploaded-image" />'
+    # Escape the src: export_image allowlists the media type and
+    # validates the base64, but the data: URL still must not be able to
+    # break out of the attribute (issue #277).
+    return f'<img src="{escape_html(src)}" alt="image" class="uploaded-image" />'
 
 
 def format_unknown_content(content: UnknownMessage) -> str:
diff --git a/claude_code_log/html/renderer.py b/claude_code_log/html/renderer.py
index 8b33a7ea..e9901f6c 100644
--- a/claude_code_log/html/renderer.py
+++ b/claude_code_log/html/renderer.py
@@ -533,7 +533,10 @@ def _format_image(self, image: ImageContent) -> str:
         )
         if src is None:
             return "[Image]"
-        return f'<img src="{src}" alt="image" class="uploaded-image" />'
+        # Escape the src: export_image allowlists the media type and
+        # validates the base64, but the data: URL (embedded mode) still
+        # must not be able to break out of the attribute (issue #277).
+        return f'<img src="{escape_html(src)}" alt="image" class="uploaded-image" />'
 
     # -------------------------------------------------------------------------
     # System Content Formatters
diff --git a/claude_code_log/html/tool_formatters.py b/claude_code_log/html/tool_formatters.py
index 08e35553..e39009e9 100644
--- a/claude_code_log/html/tool_formatters.py
+++ b/claude_code_log/html/tool_formatters.py
@@ -14,8 +14,6 @@
 HTML for display in transcripts.
 """
 
-import base64
-import binascii
 import json
 import re
 from collections.abc import Iterable
@@ -37,6 +35,7 @@
     render_user_markdown_collapsible,
     resolve_memory_body_links,
 )
+from ..image_export import is_safe_image_source
 from ..json_depth import exceeds_depth
 from ..utils import (
     ARTIFACT_FAVICON_TEXT_MAX,
@@ -1600,22 +1599,10 @@ def format_tool_result_content_raw(tool_result: ToolResultContent) -> str:
                 source = cast(dict[str, Any], item.get("source", {}))
                 if source:
                     media_type: str = str(source.get("media_type", "image/png"))
-                    # Restrict to safe image types to prevent XSS via SVG
-                    allowed_media_types = {
-                        "image/png",
-                        "image/jpeg",
-                        "image/gif",
-                        "image/webp",
-                    }
-                    if media_type not in allowed_media_types:
-                        continue
                     data: str = str(source.get("data", ""))
-                    if data:
-                        # Validate base64 data to prevent corruption/injection
-                        try:
-                            base64.b64decode(data, validate=True)
-                        except (binascii.Error, ValueError):
-                            continue
+                    # Same gate as user/assistant images: allowlisted
+                    # (non-SVG) type and strictly-valid base64.
+                    if data and is_safe_image_source(media_type, data):
                         data_url = f"data:{media_type};base64,{data}"
                         image_html_parts.append(
                             f'<img src="{escape_html(data_url)}" alt="Tool result image" '
diff --git a/claude_code_log/image_export.py b/claude_code_log/image_export.py
index 78ed4472..00c137b9 100644
--- a/claude_code_log/image_export.py
+++ b/claude_code_log/image_export.py
@@ -15,6 +15,49 @@
     from .models import ImageContent
 
 
+# Image media types we are willing to emit into a data: URL or write to
+# disk. Deliberately excludes ``image/svg+xml`` — SVG can carry inline
+# ``<script>`` and event handlers, so a data:image/svg+xml URL is a
+# scriptable XSS vector when the generated page is opened under
+# ``file://``. Shared by every image sink: user/assistant images here,
+# tool-result images in ``html/tool_formatters.py``, and plugins via
+# ``claude_code_log.plugins.is_safe_image_source`` (issue #277).
+ALLOWED_IMAGE_MEDIA_TYPES = frozenset(
+    {
+        "image/png",
+        "image/jpeg",
+        "image/gif",
+        "image/webp",
+    }
+)
+
+
+def is_safe_image_source(media_type: str, data: str) -> bool:
+    """Whether an image's media type and base64 data are safe to emit.
+
+    Guards the embedded (data: URL) and referenced (write-to-disk)
+    paths, the tool-result image path, and plugin image sinks (it is
+    re-exported from ``claude_code_log.plugins``) against two problems:
+
+    - a non-allowlisted / scriptable media type (notably SVG), and
+    - malformed base64 (which could smuggle a raw ``"`` /  ``>`` past a
+      naive interpolation, or corrupt the written file).
+
+    Returns ``True`` only when the media type is allowlisted *and* the
+    data is strictly-valid base64. Callers should fall back to a
+    placeholder (``None``) otherwise. Note this does not, on its own,
+    make the result safe to drop into HTML unescaped — the HTML sink
+    must still ``escape_html`` the final ``src`` (issue #277).
+    """
+    if media_type not in ALLOWED_IMAGE_MEDIA_TYPES:
+        return False
+    try:
+        base64.b64decode(data, validate=True)
+    except (binascii.Error, ValueError):
+        return False
+    return True
+
+
 def export_image(
     image: "ImageContent",
     mode: str,
@@ -54,12 +97,23 @@ def export_image(
         return None
 
     if mode == "embedded":
+        # Reject scriptable / non-allowlisted media types and malformed
+        # base64 before building the data: URL. Even with these guards
+        # the HTML sink must still escape the returned src (issue #277);
+        # returning None here degrades to a placeholder.
+        if not is_safe_image_source(image.source.media_type, image.source.data):
+            return None
         return f"data:{image.source.media_type};base64,{image.source.data}"
 
     if mode == "referenced":
         if output_dir is None:
             return None
 
+        # Same allowlist/validation as embedded mode: don't write a
+        # scriptable or malformed image to disk and then reference it.
+        if not is_safe_image_source(image.source.media_type, image.source.data):
+            return None
+
         tmp_path: Path | None = None
         try:
             image_data = base64.b64decode(image.source.data)
@@ -95,12 +149,17 @@ def export_image(
 
 
 def _get_extension(media_type: str) -> str:
-    """Get file extension from media type."""
+    """Get file extension from media type.
+
+    Only the allowlisted media types (``ALLOWED_IMAGE_MEDIA_TYPES``)
+    reach this in referenced mode; ``image/svg+xml`` is intentionally
+    absent because SVG is rejected upstream as a scriptable XSS vector
+    (issue #277).
+    """
     ext_map = {
         "image/png": ".png",
         "image/jpeg": ".jpg",
         "image/gif": ".gif",
         "image/webp": ".webp",
-        "image/svg+xml": ".svg",
     }
     return ext_map.get(media_type, ".png")
diff --git a/claude_code_log/plugins.py b/claude_code_log/plugins.py
index bdcbac27..11b2954d 100644
--- a/claude_code_log/plugins.py
+++ b/claude_code_log/plugins.py
@@ -44,6 +44,7 @@
         render_markdown,
         render_markdown_collapsible,
     )
+    from .image_export import is_safe_image_source
     from .markdown.renderer import safe_markdown_inline, safe_markdown_link_target
     from .utils import is_safe_web_url
 
@@ -373,6 +374,10 @@ def apply_transformers(
         # that break out of the ``(target)`` slot or smuggle attributes
         # through a downstream Markdown->HTML conversion.
         "safe_markdown_link_target",
+        # Image gate (#277): a transcript image's ``media_type`` and base64
+        # ``data`` are untrusted; a plugin building a ``data:`` URL needs the
+        # same allowlist (no SVG) + strict base64 check as the core sinks.
+        "is_safe_image_source",
     }
 )
 
@@ -381,7 +386,8 @@ def __getattr__(name: str) -> Any:  # PEP 562
     if name in _PUBLIC_HELPERS:
         # The ``safe_markdown_*`` pair (inline HTML-neutralising gate +
         # link-target percent-encoder) lives in ``markdown/renderer.py``;
-        # ``is_safe_web_url`` in top-level ``utils.py``; the others in
+        # ``is_safe_web_url`` in top-level ``utils.py``;
+        # ``is_safe_image_source`` in ``image_export.py``; the others in
         # ``html/utils.py``. Resolved lazily to keep package init acyclic.
         if name == "safe_markdown_inline":
             from .markdown.renderer import safe_markdown_inline as resolved
@@ -390,6 +396,9 @@ def __getattr__(name: str) -> Any:  # PEP 562
         elif name == "is_safe_web_url":
             # Format-neutral (shared by both renderers) → top-level utils.
             from .utils import is_safe_web_url as resolved
+        elif name == "is_safe_image_source":
+            # Format-neutral image gate → ``image_export.py``.
+            from .image_export import is_safe_image_source as resolved
         else:
             from .html import utils as _utils
 
@@ -404,6 +413,7 @@ def __getattr__(name: str) -> Any:  # PEP 562
     "MessageTransformer",
     "apply_transformers",
     "escape_html",
+    "is_safe_image_source",
     "is_safe_web_url",
     "load_transformers",
     "render_markdown",
diff --git a/dev-docs/application_model.md b/dev-docs/application_model.md
index b314c1f2..c5d7a4df 100644
--- a/dev-docs/application_model.md
+++ b/dev-docs/application_model.md
@@ -566,6 +566,17 @@ format-agnostic: HTML and Markdown both call into it. Three modes
 - `referenced` — write the image to disk next to the output and
   embed a `src=` reference.
 
+`embedded` and `referenced` both refuse an image unless
+`is_safe_image_source` passes: the media type must be PNG, JPEG, GIF
+or WebP (never the scriptable `image/svg+xml`) and the data strictly
+valid base64, otherwise the image degrades to the placeholder. The
+transcript's `media_type` and `data` are untrusted (an image can come
+back from a tool, an MCP server or a web page), and a crafted
+`media_type` used to break out of the `<img src>` attribute (issue
+#277); the HTML sinks also `escape_html` the final `src`. Tool-result
+images go through the same gate, and plugins get it from
+`claude_code_log.plugins` ([plugins.md § 4.2](plugins.md#42-security-conscious-rendering)).
+
 Default is `embedded` for HTML (single self-contained file) and
 `referenced` for Markdown (keeps the `.md` text small and lets
 images live as separate PNGs alongside).
diff --git a/dev-docs/plugins.md b/dev-docs/plugins.md
index 76851236..201ca4a4 100644
--- a/dev-docs/plugins.md
+++ b/dev-docs/plugins.md
@@ -289,11 +289,12 @@ moves to the next ancestor.
 
 ### 4.1 Plugin-facing helpers
 
-Five helpers are re-exported from `claude_code_log.plugins` for use
+Seven helpers are re-exported from `claude_code_log.plugins` for use
 in `format_html` / `format_markdown` / `title` methods. The re-export is
 the stable plugin API; the underlying implementation (in
-`claude_code_log/html/utils.py`, `claude_code_log/markdown/renderer.py`
-and `claude_code_log/utils.py`) may move or be renamed.
+`claude_code_log/html/utils.py`, `claude_code_log/markdown/renderer.py`,
+`claude_code_log/image_export.py` and `claude_code_log/utils.py`) may
+move or be renamed.
 
 ```python
 from claude_code_log.plugins import (
@@ -303,6 +304,7 @@ from claude_code_log.plugins import (
     safe_markdown_inline,
     is_safe_web_url,
     safe_markdown_link_target,
+    is_safe_image_source,
 )
 ```
 
@@ -314,6 +316,7 @@ from claude_code_log.plugins import (
 | `safe_markdown_inline(text)` | `(str) -> str` | Interpolating transcript-derived text into a Markdown **inline** fragment in `format_markdown` (a link label, a list item, inline prose) — the Markdown-output path emits `format_markdown` verbatim. Entity-escapes raw HTML tags while preserving Markdown formatting. |
 | `is_safe_web_url(url)` | `(str) -> bool` | Gating a transcript-derived URL before making it clickable — an `href` in `format_html` or a `[text](target)` destination in `format_markdown`. Escaping cannot neutralise a hostile scheme (`javascript:alert(1)` survives `escape_html` intact), so emit the link only when this returns True and degrade to inert escaped text otherwise. http(s) whitelist, fails closed. |
 | `safe_markdown_link_target(url)` | `(str) -> str` | Encoding a URL you are about to place in a Markdown `(target)` slot — a `[text](target)` link or `![](target)` image in `format_markdown`. An `is_safe_web_url`-accepted URL can still carry `"`/`'`/`<`/`>`/space/parens that break out of the destination or smuggle attributes once a downstream viewer converts the Markdown to HTML; this percent-encodes them. Use **after** the scheme gate, on the target only (label the link with `safe_markdown_inline` text). |
+| `is_safe_image_source(media_type, data)` | `(str, str) -> bool` | Gating a transcript-derived image before you build a `data:{media_type};base64,{data}` URL for an `<img src>` (or a Markdown `![](…)` target). True only for an allowlisted type (`image/png`, `image/jpeg`, `image/gif`, `image/webp` — never the scriptable `image/svg+xml`) with strictly-valid base64; render a placeholder otherwise. The same gate the core image sinks use. It does not escape: still `escape_html` the finished URL in an HTML attribute. |
 
 The reference plugin's
 [`tool_communicate_result.py`](../test/_plugins/clmail/src/claude_code_log_clmail_test/transformers/tool_communicate_result.py)
@@ -347,6 +350,7 @@ output paths:
 | `title()` return | `escape_html(value)` — the HTML header emits it via `\| safe` (no core escaping); the Markdown heading is auto-gated by the core |
 | URL made clickable (`href` attribute) | gate on `is_safe_web_url(url)` first, **then** `escape_html(url)` for the attribute text — the scheme check and the escaping guard different attacks |
 | URL made clickable (Markdown link/image target) | gate on `is_safe_web_url(url)` first, **then** `safe_markdown_link_target(url)` for the `(target)` slot — guards against attribute-smuggling through the downstream Markdown→HTML pass |
+| Image `data:` URL (`<img src>` or Markdown `![](…)`) from a transcript `media_type` + base64 `data` | gate on `is_safe_image_source(media_type, data)` first (allowlist without SVG, strict base64), **then** `escape_html(data_url)` for the HTML attribute — a crafted `media_type` such as `png"><script>…` otherwise closes the attribute (#277) |
 
 Notes:
 
diff --git a/test/test_image_export.py b/test/test_image_export.py
index 530771b7..d4787bc4 100644
--- a/test/test_image_export.py
+++ b/test/test_image_export.py
@@ -135,6 +135,110 @@ def test_unsupported_mode_returns_none(self, sample_image: ImageContent):
         assert result is None
 
 
+# Minimal valid PNG (1x1 transparent pixel), shared by the security tests.
+_VALID_PNG_B64 = (
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR4nGNgYGBgAAAABQABpfZFQAAAAA"
+    "BJRU5ErkJggg=="
+)
+
+
+def _image(media_type: str, data: str = _VALID_PNG_B64) -> ImageContent:
+    return ImageContent(
+        type="image",
+        source=ImageSource(type="base64", media_type=media_type, data=data),
+    )
+
+
+class TestExportImageSecurity:
+    """Guards against the embedded/referenced-image XSS vector (issue #277).
+
+    ``media_type`` and ``data`` are unvalidated strings parsed from
+    transcript JSON, and can carry content that did not originate from
+    the user (tool/MCP-returned images, fetched web content). Without
+    an allowlist + base64 validation, a crafted media type could break
+    out of the ``<img src>`` attribute.
+    """
+
+    def test_embedded_rejects_attribute_breakout_media_type(self):
+        """A media type crafted to break out of the src attribute yields no data URL."""
+        hostile = _image('png"><script>alert(document.domain)</script>')
+        assert export_image(hostile, mode="embedded") is None
+
+    def test_embedded_rejects_svg(self):
+        """SVG is scriptable; embedded mode must not emit a data:image/svg+xml URL."""
+        svg = _image("image/svg+xml")
+        assert export_image(svg, mode="embedded") is None
+
+    def test_embedded_rejects_invalid_base64(self):
+        """Malformed base64 data is rejected rather than emitted verbatim."""
+        bad = _image("image/png", data='not"base64><script>')
+        assert export_image(bad, mode="embedded") is None
+
+    def test_embedded_allows_valid_allowlisted_types(self):
+        """The four allowlisted types with valid base64 still produce data URLs."""
+        for mt in ("image/png", "image/jpeg", "image/gif", "image/webp"):
+            result = export_image(_image(mt), mode="embedded")
+            assert result is not None, mt
+            assert result.startswith(f"data:{mt};base64,")
+
+    def test_referenced_rejects_svg(self, tmp_path: Path):
+        """Referenced mode must not write a scriptable SVG to disk and link it."""
+        svg = _image("image/svg+xml")
+        result = export_image(svg, mode="referenced", output_dir=tmp_path)
+        assert result is None
+        assert not (tmp_path / "images").exists() or not list(
+            (tmp_path / "images").iterdir()
+        )
+
+    def test_referenced_rejects_invalid_base64(self, tmp_path: Path):
+        """Referenced mode rejects malformed base64 instead of writing garbage."""
+        bad = _image("image/png", data="!!!not-base64!!!")
+        result = export_image(bad, mode="referenced", output_dir=tmp_path)
+        assert result is None
+
+    def test_html_sink_escapes_hostile_media_type(self):
+        """End-to-end: the HTML <img> sink never emits an unescaped breakout.
+
+        Even if a future change let a hostile media type through
+        export_image, the HTML formatter must escape the src so no
+        ``">`` can close the attribute/tag. Here the hostile type is
+        rejected upstream (rendering a placeholder), but we also assert
+        no live ``<script>`` or attribute-closing ``">`` leaks.
+        """
+        from claude_code_log.html import format_image_content
+
+        hostile = _image('png"><script>alert(1)</script>')
+        html = format_image_content(hostile)
+        assert "<script>" not in html
+        assert '"><script' not in html
+
+    _BREAKOUT_SRC = 'data:image/png;base64,AAAA"><script>alert(1)</script>'
+
+    def test_assistant_sink_escapes_src(self, monkeypatch: pytest.MonkeyPatch):
+        """The escaping layer on its own: with the gate bypassed, a quote in
+        the src still cannot close the attribute."""
+        import claude_code_log.html.assistant_formatters as formatters
+
+        monkeypatch.setattr(
+            formatters, "export_image", lambda *a, **k: self._BREAKOUT_SRC
+        )
+        html = formatters.format_image_content(_image("image/png"))
+        assert "<script>" not in html
+        assert 'src="data:image/png;base64,AAAA&quot;&gt;&lt;script&gt;' in html
+
+    def test_renderer_sink_escapes_src(self, monkeypatch: pytest.MonkeyPatch):
+        """Same for the renderer's export-mode-aware sink."""
+        import claude_code_log.image_export as image_export
+        from claude_code_log.html.renderer import HtmlRenderer
+
+        monkeypatch.setattr(
+            image_export, "export_image", lambda *a, **k: self._BREAKOUT_SRC
+        )
+        html = HtmlRenderer()._format_image(_image("image/png"))
+        assert "<script>" not in html
+        assert 'src="data:image/png;base64,AAAA&quot;&gt;&lt;script&gt;' in html
+
+
 class TestReferencedModeAcrossRenderPasses:
     """Regression: the combined and per-session passes share one images/ dir.
 
diff --git a/test/test_image_paste_ids.py b/test/test_image_paste_ids.py
index f74c1945..ccf82367 100644
--- a/test/test_image_paste_ids.py
+++ b/test/test_image_paste_ids.py
@@ -430,8 +430,8 @@ def _entry(
     ) -> dict[str, object]:
         if content is None:
             content = [
-                self._block(f"ZZFIRSTZZ{_PNG_1X1}"),
-                self._block(f"ZZSECONDZZ{_PNG_1X1}"),
+                self._block(f"ZZFIRSTZZZZZ{_PNG_1X1}"),
+                self._block(f"ZZSECONDZZZZ{_PNG_1X1}"),
                 {"type": "text", "text": "look at [Image #2]"},
             ]
         return {
diff --git a/test/test_plugin_xss_api.py b/test/test_plugin_xss_api.py
index bb826be4..9e86d3b0 100644
--- a/test/test_plugin_xss_api.py
+++ b/test/test_plugin_xss_api.py
@@ -36,6 +36,7 @@ def test_helpers_importable_and_in_all(self):
             "safe_markdown_inline",
             "is_safe_web_url",
             "safe_markdown_link_target",
+            "is_safe_image_source",
         ):
             assert hasattr(plugins, name), name
             assert name in plugins.__all__, name
@@ -61,6 +62,20 @@ def test_is_safe_web_url_behaviour(self):
         assert not is_safe_web_url("HTTP://example.com")  # fails closed, by design
         assert not is_safe_web_url("")
 
+    def test_is_safe_image_source_behaviour(self):
+        from claude_code_log.plugins import is_safe_image_source
+
+        png = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR4nGNgYGBgAAAABQABpfZFQAAAAABJRU5ErkJggg=="
+        for media_type in ("image/png", "image/jpeg", "image/gif", "image/webp"):
+            assert is_safe_image_source(media_type, png), media_type
+        # SVG is scriptable; anything off the allowlist fails closed (#277).
+        assert not is_safe_image_source("image/svg+xml", png)
+        assert not is_safe_image_source('png"><script>alert(1)</script>', png)
+        assert not is_safe_image_source("IMAGE/PNG", png)
+        # Strict base64: a quote or angle bracket in the data is rejected.
+        assert not is_safe_image_source("image/png", 'AAAA"><script>')
+        assert not is_safe_image_source("image/png", "!!!not-base64!!!")
+
     def test_safe_markdown_link_target_behaviour(self):
         from claude_code_log.plugins import safe_markdown_link_target
 
diff --git a/test/test_steering_queued_command.py b/test/test_steering_queued_command.py
index a5bb8fa2..b85163cb 100644
--- a/test/test_steering_queued_command.py
+++ b/test/test_steering_queued_command.py
@@ -676,7 +676,7 @@ def test_non_contiguous_paste_ids_resolve_by_id_not_position(self):
         # (`:first-child`, `--text-secondary`) *before* any message content — so
         # the ordering assertion below compared two CSS offsets and was true
         # whatever the resolver did.
-        alpha, beta = _image_block("ZZBLOCKALPHAZZ"), _image_block("ZZBLOCKBETAZZ")
+        alpha, beta = _image_block("ZZBLOCKALPHAZZZZ"), _image_block("ZZBLOCKBETAZZZZZ")
         html = _render(
             [
                 _user("u1", None, "start"),
diff --git a/test/test_user_renderer.py b/test/test_user_renderer.py
index d1c74b53..41609fac 100644
--- a/test/test_user_renderer.py
+++ b/test/test_user_renderer.py
@@ -244,11 +244,11 @@ def test_numbered_image_references_control_image_placement(self):
         """
         first = ImageContent(
             type="image",
-            source=ImageSource(type="base64", media_type="image/png", data="first"),
+            source=ImageSource(type="base64", media_type="image/png", data="first000"),
         )
         second = ImageContent(
             type="image",
-            source=ImageSource(type="base64", media_type="image/png", data="second"),
+            source=ImageSource(type="base64", media_type="image/png", data="second00"),
         )
         content_list: list[ContentItem] = [
             first,