the whole over all pic is kinda "dark" u should try lightening it up slightly.
i like the purple part in the background as thats the same as his eye part on his mask.
if u manage to lighten the background slightly try a bit of transparency on the render, i would say 80-85% in my oppinion (but im not great at it)
also add a black border that might finished the edges off slightly
[edit] just having a second look, the pic its self is slightly "small" and compact, try and add 50 pixles on the bottom. just to "open" the image a bit and show off a little "extra" the black on the right, doesnt seem to fit as it goes from "quite light" to dark quite quick, if it was light to dark(still slightly visable) not black that would look slightly better and less dull