It looks absolutely wonderful, far better than anything I could do. but the last one looks like you forgot the angle you were working with when you hit the bottom of the hand is all.

If you were to just cover up the bottom of the hand, across at the top of the knuckles), it looks as if you're viewing the open side of the palm making it natural for all the fingers to fall in view as opposed to just the thumb and the index. I could just be looking too much into it, but that's the best I could think how to explain why the last one's hand felt unnatural to me.

The last one looks like a left arm with a right hand. I don't think it's natural for the right arm to bend that way.

Wow. What photoshop brushes do you use? That looks nearly like graphite, but again with the hand.

Nice study, be careful of that arm on the far right(left arm), it looks like it has a right hand.