First off, what I love is your character right up front. The way she looks with her expression, the symbol on her face fitting with the scene perfectly, as well as the leter and flashlight, looking absolutely natural in her hands. Her expression shows the intensity of the situation and how dire she wants to escape. The only point I have to point out (I'm so sorry to nitpick >_<) is her left arm, it is slightly long to me, but, other than that, it looks quite awesome! :3
Next, the background! It looks so similar to the game, and it shows the darkness, and the scariness of the game quite on the money. The tree, as well as the little notch for where the note was, wow. Awesome. It gives off the vibe to me as the actual Slender. And your slenderman is FANTASTIC. Just blending in and if it wasn't the face, you couldn't tell it from all the other trees. I would have liked to have seen the background just a little darker, to give a much more scary vibe XD But, that's just me nitpicking again.
All in all, this is just awesome. I absolutely love this picture :3 Great job!