I went through this once before, and tried cleaning it up, but recently I moved my audio library to another hard drive, and proceeded to import the audio files. After doing so, I'm finding some issues with the cover art files being associated with wrong songs. It appears that MJ is trying to assign an common image to many different files. There's even some cases when I've got an entire album, with a cover.jpg in the folder, and in MJ half of the songs have the correct cover art, and the other half of them are some hacked-up photo.
This has happened before, when I would copy the cover art image to the clipboard, and paste it, later on I would find this cover art image which i copied embedded in other files, and I never did it!
I tried yesterday going through and deleting the cover art associated with these audio files (there was probably 60+), but as I see again today this cover art image is now back but is assigned to numerous other files (probably 30+ this time, different audio files tho - and i didn't miss them yesterday).
Any ideas on how to stop this from happening? Or why this is happening?