[KevinH]

Missing images are replaced by image placeholders in Preview visually. So a visual scan in Preview can be fast and effective and part of good proofreading practices.

There is also a contributed plugin that can identify broken links. A missing image should be an example of one.

Of course, your best bet would be to run epubcheck as it will identify broken/missing images.

You can also use regular expression find and replace to get a list of img tag src values to inspect quite easily.

Hope something here helps.