Since you haven't gotten anything back on this from somebody that knows the exact answer....
There is a folder somewhere that stores these artist images. If you find it and delete these images via Windows Explorer or similar means, MC will revert to the album art images. I do not know off the top of my head where this directory is, but the fact that I know of its existence means that the exact location is listed around these forums
somewhere....
Maybe you'll get lucky and somebody with more precise knowledge will show up before you decide to dive into the forum search.
-Jon