Sorry, I was a bit unclear. I messed up Thumbnails which created for faster viewing CoverArt preview inside MC and the CoverArt itself.
What I was referring to in my previous post were thumbnails. They're NOT stored inside a library backup and so have to be recreated after a library restore. They are created using the original CoverArt or by taking screenshots out of videos and are much smaller and therefor faster to load and display.
The CoverArt itself also doesn't get backuped by MC, because MC only saves the LIBRARY, not the files them self. As so, neither your mp3, pictures or videos managed with MC nor the CoverArt gets backed up with a Library Backup.
But of course storing your cover art inside a file is NOT the only way to store your cover art. You also can use a directory containing all pictures used as covers.
Hope that clears things up a bit. Sorry for being that unclear before.