BATMAN! :D or the Joker. Also my sister did this fantastic flash of a brain and the lower part of a skull melded together - I can ask her to send it to you if you want....
The Red Skull (extra awesome if there's the Winter Soldier too). The SpiderWoman of Xxxholic. Sweeney Todd slitting the throat of someone. Hm. Darth Vader. The Emperor. Jason Todd. A burning teddy bear. Boris Karloff in Frankenstein. (can't go wrong with the classics) The tarot card "The Hanged Man", super creeperified. Hmm. I'm running dry, too. Oh! The rat creatures from Bone! Though it's more fun than creepy.
I hope I sent enough your way to get you thinking!