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....
![Grin ;D](https://yabb.jriver.com/interact/Smileys/classic/grin.gif)
Maybe you'll get lucky and somebody with more precise knowledge will show up before you decide to dive into the forum search.
-Jon