I came here to post a question about cover art, but I have found a solution.
So sometimes, if I have two different albums with the same name, gmb will show the same cover art for both. No matter what I do — renaming the album, removing and re-adding songs, even changing the filenames — nothing will get the album art to show properly.
You can right click an album cover and replace the picture with a file for that album, but this requires that cover art image to stay in place. If you keep your art embedded in the MP3s, this isn't a great solution.
However, you can right click to set the album art, and then choose one of the mp3s themselves as the source. As long as your album art is embedded correctly, this will set the art to what it's supposed to be.