[KevinH]

That can easily happen if the later xhtml files are not in the same directory as the earlier xhtml files meaning the url must be different to find the image file.

Go into Sigil Preferences and enable showing full book paths in BookBrowser and then post a screen cap of the resulting BookBrowser window so we can see the actual paths used for the xhtml file and the image file for one instance it does not work.

Using Sigil insert media tool can be helpful as it will always properly create the proper url for wherever you put the files inside the epub.