Comment 2 for bug 120975

What I usually do is find ~/.gnome2/rhythmbox/covers and set its permissions to "access files" only.
I realize it probably still downloads the covers, just doesnt save them, but I'm rarely missing a "folder.jpg", so it only does that in extreme situations.
So if you're just trying to prevent it from downloading it accidentally (maybe in the time it takes for you to put a better-looking, bigger cover in the album directory), it works quite good.