Both are influenced by the "tool" posters that Saw & Hostel made so popular.
But why is wishbaby's pacifier bloody? IMDB says the plot to wishbaby is : "A troubled young black teenager Maxine rescues an eccentric old lady Eve from an unhappy slap by a mixed-race gang of teens and is rewarded for her act of kindness with the gift of a terrible secret: how to make and operate a wishbaby." My guess is that the wishbaby is some kind of voodoo zombie doll that feeds on bloody run-on sentences.
With the Donkey Punch poster, the motor is framed to look like a flopped over, open-mouthed skull, while the bottom/propeller is all bloody. Everyone knows what a donkey punch is, right? Well the poster does it's best to emulate it. And yes, one of those occurs in the movie. And yes, she dies from it. And yes, the movie takes off from there... But no, I haven't seen it yet. Until I do, I'm keeping Teeth as my frontrunner for worst sexified horror film of 2008.