Use the pen tool or lasso tool to extract the snake head...dunno what you did here, but it's not good...

nice idea -- check the masking the snakes (light spot under the jaw seems odd)

Yes, masking is a better way to do the contour of the snakes heads. The edges are too evident, even at first look.

You can also use polygonal lasso tool with a 1 px feather. the edges are really sloppy in high res.

Nice work but I do agree with the masking suggestions that others have made. Would make a good idea an excellent entry. Good luck to you