This is marked as fixed in the next release.
In the next release, a new VaultWiki: Aesthetics option:
Lazy load wiki attachment images?
0 - No
1 - Yes, using fixed heights, loading indicators, and error notices. (default)
2 - Yes, using browser-default size before loading and on errors, with a throttle.
When there is a problem loading the image:
#1 will use CSS to render a relevant error icon, and the alt text is changed to a relevant error message (like "You do not have permission...", "Resource not found", etc).
#2 will just use the browser default for failed images, which might be a 0x0 render, or might be a small red X. As I discussed above, since this would make the document height unknown, we have a lazy-load queue that can only contain 10 images at a time, recalculating the height after each image. This prevents all images from loading at once when later images might end up off-screen after an earlier image loads.