Pieter Bruegel's
The Triumph of Death was first introduced to me as the artwork on Black Sabbath's Greatest Hits.
Its a striking painting with so much going on, much of it being mutilation, rape and torture.
A truly grim yet fascinating piece.
You can really explore the intricate detail by enlarging the pic in its Wiki page.
http://en.wikipedia.org/wiki/The_Triumph_of_Death