Someone reported this happening a while ago. But, I haven't seen it on any of my devices. I don't remember if there was a solution. The suggestions to try would have included:
- Using account repair on the Device information page of the settings.
- Check the database on the device has no errors.
- Signing out and back into the account.
- A factory reset. But, only if absolutely nothing else fixed it.
Other thoughts would be on how much space you have left on the device. Kobo likes to have between 100 and 200MB free for firmware updates. But, I've only seen cover image issues when the storage was actually full.
If the file system is corrupt, it could lose the images. The issue might be that while the cover images are in the cache, they are OK, but, if there was a problem writing to the physical storage, it wouldn't be able to find them later. Checking what is in ".kobo-images" and seeing if the cover images are actually there might tell us something. Checking the file system when connected to a PC might help.
You could try turning on devmode and looking at the logs produced. There might be something in that to show what is happening.