I'm a pretty big fan of Aiden Hughes. He's done a lot of the cover art for KMFDM.

http://www.bruteprop.com/

I particularly enjoy these two:

http://www.bruteprop.com/v1/gallery/img/holdup.gif

http://www.bruteprop.com/v1/gallery/img/driving_man.gif

And the one I used for my sig, of course.