Set the blend mode on the skull to "Overlay" and see how that looks. Also, the text is hard to read; maybe a soft white outer glow? Border looks good. I would suggest making the inage on the left much larger. You don't need to have his whole body in there, just maybe face and shoulders. If you want the gun to be visible, then duplicate the layer and blend it into the b/g.
I did that in this sig because I wanted the boobs to be in there, lol: