[retiredbiker]

Two things.

One, when you change the cover inside the book using the Editor, it does not change the external copy of the cover stored with the book in the book folder. That "backup copy" will hang around and come back to haunt you if you leave it.

Two, there is a setting in the Editor Preferences: in the "integration with Calibre" section, uncheck "Update metadata embedded in the book when opening". This is probably what is causing the cover to get restored from the folder when you open the editor again. Note this is Editor Preferences, not the general ones, get to it from within the editor.

The safest way to put your edited cover into the folder as the external copy is, after the edit, open Edit Metadata for that book. Then in the format control box at the top right, highlight the newly edited epub and then click the blue box. The tool tip says "set the cover for the book from the selected format"...it is actually setting the external copy.

If you don't correct that external copy, It will also come back if you Polish the book, and probably in other places as well, depending on various settings.

Yes this is confusing!