This is a good picture, anatomy can be sooooo difficult, its a lot easier if you have models to work with. Poser 4 is a great 3d program that kinda helps you understand movement better, in people and animals.
Shading below where the ribs are would help them stand out more, and go over top the shadow with an eraser to bring out the highlight. This gives the picture a deeper feel...The legs actually look ok to me. The breasts may actually be a bit too high, but that may just be me, but they come across as kind of flat(meaning no depth) But I'm no expert, and am not criticizing. Just constructive advice. The piece has a lot of emotion to it, and I commend you for that. Art isn't special without the feeling behind it. Keep up the good work! :)