Just started trying some morph work. Let me know what you think...

I would suggest using pictures that give you more room to work with. Even if you aren't trying to make the real big its always nice to have more room to work with. I personally like to always see the girls face as well, so that makes the middle one the best IMO. You may also want to try using a softer paint brush when doing your layer mask so the belly doesn't have such a hard edge.

Just because I liked the expression on her face, and because maybe it would help you, I did my own version of the middle girl you posted: