Image not found
An image reference points at a file that isn't there. This isn't a Keystone check — Pandoc, the tool underneath, can't fetch the file and warns. The build continues, but the image is missing from your book.
What it looks like

With no assets/missing.png, the build prints (PDF):
[WARNING] Could not fetch resource assets/missing.png: replacing image with description
and EPUB:
[WARNING] Could not fetch resource assets/missing.png: PandocResourceNotFound "assets/missing.png"
What it means
Pandoc resolves image paths as it builds. A path that doesn't resolve can't be fetched, so Pandoc drops the image with a warning and carries on — in PDF it substitutes the image's alt text ("replacing image with description"), in EPUB it leaves the reference unresolved. Because it's a warning, a lenient build still succeeds with the image simply gone. Under strict mode it fails the build — which is how you catch a missing image before you publish.
How to fix
- Check the path's spelling and that the file exists under
assets/. - Image paths are relative to your project; see Figures & images.
- Build strict before publishing so a missing image stops the build instead of shipping a gap.
Related
- Undefined citation — the other common tool-origin warning that a lenient build lets through and strict mode catches.