Some time ago I hacked together a little python script to do the job (disclaimer: it was just a couple of minutes work so no warranty

) that gets pictures from last.fm and writes them to Artist/folder.jpg.
Save as getartist.pyYou need to install
Python 2.6 and the
Python Imaging Library (PIL).
You also need to fetch
pylast.py. Put the pylast.py in the same folder as getartist.py (or
install pylast properly) and tweak getartist.py as you like (in particular START_DIR when you don't run it from your root music folder).
You should also change the API_KEY (
get your own one) but it should work as it is.
It uses last.fm to fetch the artist images and you need to have your music stored as START_DIR/Genre/Artist/.
I apply a stupid heuristic to make the image quadratic (just use the maximum square in the center) and scales it down (or up) to 500x500 px.
Maybe it's useful for you - it did the job for me.