I could be wrong, but to my knowledge, the database retains no reference to any album art, either “files on the side” e.g.
cover.jpg, nor art embedded in the music files themselves. At least, I couldn’t find any such references when digging through my whole music collection of tens of thousands of songs, which has plenty of album art embedded in files, and a few files on the side too. So, I think you can just delete the ones you don’t want to keep.
If you want to programmatically delete older album art files, leaving only the newest ones, you could write a shell script for that… do you need guidance on how to do that?
I think that will depend on whether you have enabled the embedart plugin.