I do it like this: (Yes, it's painful and slow)

Manually go through each directory and rename all the album art by placing a dot "." in front of the file name.

Eg:
/home/user/MyDocs/.sound/music/album/album_art.jpg
I will change this to be
/home/user/MyDocs/.sound/music/album/.album_art.jpg

It still shows up in the media player, but doesn't show up in the gallery.

It's a pity tracker isn't more configurable. (Maybe it is, I just haven't figured it out yet)