Don't post a peice in double-sized pixels, please. We have a tool for that.
Get rid of that purple color in the shadow, make back of skull 1 px smaller, lenghtwise.
It looks like you AAed this to be on a dark background. Outline the peice in black, and getr rid of the shadow.
On the plus side, this peice looks good anatomically, and your light is good.
It is always harder to fool, than to appear foolish, ya know : /