diff --git a/scripts/mutation_test.py b/scripts/mutation_test.py
index fb6bfa1..7d49dbb 100755
--- a/scripts/mutation_test.py
+++ b/scripts/mutation_test.py
@@ -286,9 +286,25 @@ def copy_tree(src, dst):
     # other reason -- it was driven in place out of deps/basicinterpret -- and
     # now lives in tests/reference/, which comes along with the rest of the tree.
     # Only vendored Windows DLLs are dead weight, hence "*.dll".
-    ignore = shutil.ignore_patterns("build*", ".git", "*.o", "*.so", "*~",
-                                    "#*#", "*.iso", "*.png", "*.gcno", "*.gcda",
-                                    "*.dll")
+    #
+    # **PNGs are ours or theirs, and the difference matters.** This used to drop
+    # every one of them, which was right when the only PNGs in the tree were
+    # SDL_image's and libpng's test corpora -- 39 MB that nothing here reads.
+    # docs/images/ changed that: docs_examples asserts every `screenshot=` block
+    # has its figure, so dropping them made the *baseline* fail and no mutation
+    # run could start at all. Keep the project's thirteen; still drop the
+    # dependencies' thousands.
+    patterns = shutil.ignore_patterns("build*", ".git", "*.o", "*.so", "*~",
+                                      "#*#", "*.iso", "*.gcno", "*.gcda",
+                                      "*.dll")
+    depsdir = os.path.join(os.path.abspath(src), "deps")
+
+    def ignore(directory, names):
+        skip = set(patterns(directory, names))
+        if os.path.abspath(directory).startswith(depsdir):
+            skip.update(n for n in names if n.endswith(".png"))
+        return skip
+
     shutil.copytree(src, dst, ignore=ignore, symlinks=True)