This is one of the more common topics on the paranormal board of 4chan. I'll post the original creepy pasta here soon. For now, pictures. For understanding the pictures, Slenderman is this vaguely humanoid thing that is very tall and has either multiple limbs or simply limb like protrusions from its body, and it supposedly causes children to disappear, killing any adults who stand in its way as it attempts to take them wherever it takes them. A lot of these are artists' representations, but a few are alleged photographs.
This one's a little hard to tell; he's behind the long banner hanging from the fence.