FOFBrainz is a utility that looks through the Frets on Fire song directories and looks for missing cover art. When a song is found that does not have cover art (or a "label" in Frets on Fire talk), the utility goes to the MusicBrainz database to get it.
This utility is still in its alpha phase. Testing and feedback are highly encouraged!
Go ahead and visit the FOFBrainz project home page.