Ditto ADrawingMan
I want to add that  you should work more with the skull, you're cutting a good proportion of it. 
 

Imho I think there's still a lot you could add to this piece, for example, you should try to take a bit more advantage of dithering if you're just working with these two colours.

Right now you just have a bit of weird dithering on the nose, it looks a bit inconsistent atm, you should use dithering to describe more facial featuers such as the cheek bones (or dents in the cheek, whutever those are called), shadows that the hair cast etc...You should also try to give more of a light source which would really help with all the details. Just remember, dithering is your friend for a piece with two colours :P 

Imo, I also think that the eyes are too close to the forehead, but that may just be me.

Not exactly an in depth critique, but hopefully it helps in one way or the other : )