I've "heard" that this id3 tagger http://massid3lib.sourceforge.net/ (free) is very good, recommended & convenient to use because you can scan mp3's on your desktop very easily. However, I notice it hasn't been updated for a while so I don't know how it performs with album art. But it can hopefully add or remove album art ok. You may want to delete or add back the album art to the actual song file (& not separated as it is)! (I've only used it a couple of times!)

Personally I like album art & have added it myself. If you have song with album art one of an album & you have the entire album, then it's easy to copy the art across to the other songs in one foul swoop!

BTW is the song an mp3 or wma or something? I've heard wmp cannot handle some types of id3 tags.