The picture isn't edited, and it's definitely not Gmod. They swapped Clem's model with Kenny's, and the Stranger's with Duck's, but kept the same animations... which is why Kenny looks so creepy in that position.
I remember hearing about Telltale not wanting to release their engine to the public because they didn't want anything fan-made to be created(With the recent incident on their forum, I can see why). That.. doesn't mean the game is unhackable- no game is unhackable. You just need the right tools. This partially worries me(again, the recent forum incident), and kind of makes me happy(there's a good side to everything).