[KevinH]

I can think of a few things that might cause that behaviour'

1. Are you by chance leaving Sigil open for days at a time while editing the same book? (i.e. not saving the epub and quitting out of Sigil until ready to use it again)?

Sigil stores books opened into its own tmp folder which defaults to the system tmp folder.

But some operating systems will walk files that exist in tmp folders and if it deems that file unused or old, the os will simply delete them under Sigil's nose.

There is a Sigil Preference setting that allows you to set a different tmp folder workspace in that event.

But there are Windows settings that can disable this "os feature" as well.

2. Additionally Anti-virus and Defender software can sometimes interfere with Sigil and it needs to be told Sigil is safe to allow Sigil to work properly.

3. And how exactly are you adding those images to Sigil? Are you using the "Add Existing ..." menu item or using some other way? If manually adding files to Sigil's tmp folder that could prevent the opf manifest from being properly updated.