Really? Looking at your photos, you really have an eye for proportional spacing and such, I would think it wasn't a problem for you. O:
Um.. try starting off your bodies with a line. Like a noodle. Think of it as a spine, position it how you think your characters back will look, then draw the body shapes from there.
Look at the blank piece of paper the way you would look through a lens, then imagine the body and let it come alive for you. Try not to be fussy and focus on one thing, let it come to you. It probably wont come the first time, but keep trying.
I hope that helps.