Whooooa! That's REALLY scary. I like that the picture tells a story, too! Lots of people just stick Slender Man into an empty landscape shot, which is spooky, but not as much as it could be.
I think if he was just a head taller, it would be 100% scarier. You can't really see his feet anyway, so you could probably just pose on some dark-colored blocks without having worry about stilts/longer dress pants.