Back in June the 8th 2009, a ‘paranormal pictures’ photoshop contest thread was launched on the Something Awful forums.
This idea to create scary pictures, and even to use them to trick paranormal forums, became entertaining among users there who started altering pictures and adding fake ghosts and other strange figures, typically appending fake stories to make it look more realistic.
On June the 10th, user Victor Surge wrote this post. It was the first known mention of the Slender Man as a creature who seemingly stalked children. That was also where the first two altered photos of him were created.