I'd go with the first one, to be honest. She has decent bone and appears to have the best conformation (although the pic isn't the best).
The second one is good, but as Ripper said, is a little tied in behind the knee and is VERY posty through the hocks.
The third one's knees/forelegs kind of worry me. Not sure what I'm seeing, but her knees almost look like they buckle backward (severely over at the knee, maybe)? Her back also looks fairly long and she's REALLY light on bone.
The fourth one is decent, but again, light on bone. S/he is also light on bone and a wee bit long in the back.
I'd be interested to know the breeding on the last two. The fourth one especially looks VERY halter bred to me.
I really don't like any of them. I guess if, in this buyer's market you can't find anything else, buy the one that listens best.
I don't get it--my DD's spent a little over $150 for my Nikon COOPIX S2600 pocket digital camera. It takes pictures like this~
Notice the clarity. I can shoot until the cows, chickens AND horses come home and pick the best. Why do we see such abysmal pictures of horses for sale?!?