I do gore photoshops, but don't know any site that would let me post them. Can't find a single law against it, unless of course you "threaten" someone (like Photoshop a celeb cut in half with half their face ripped off and caption it 'what I'm going to do to you')
But yeah, I've done requests for friends for Halloween, one of my friends ripped almost in half "by her hung boyfriend" like the waitress in family Guy, some celebs who survived in horror movies as a post credit death..

I'd love to post them somewhere.