I found that it was necessary to BOTH delete the plugin as ethan.iron described AND delete the "Unknown Artist" image folder as Maccleduff described.
Presumably deleting the plugin only stops GrowlTunes from downloading new album art. If, like me, you have been staring at that revoltiong bloody shoe UnterNull thing for months, than you likely already have many copies of it on your hard drive, which is why you have to also delete the Unkown Artists folder.
Thanks everyone!