Here we go.
The Background first: I very much like the way you did the background. The red trees against the black BG really give an eerie feel to the picture.
The Super Sexy Slendy: His torso seems a bit long, but- (Wait. He is Slenderman... never mind that O.o) I think the shading on either sides of the abdominal muscles is quite excessive, but around the neck area it is PERFECT! I like the technique used in doing the pants. They look very realistic. His head seems a bit small in proportions/comparison to his body, but maybe that's just me.
So, all in all, 5 stars for me. Mainly cause I have done one (not published, or as good as this...) and love Sexy Slender pics.