I agree with Hedgie's observations posted above, but what I see a different cause and effect; I think that your hip joint is not working properly for you in your two point and that's causing the other position problems.
The first and fourth photo, being taken head on, aren't terribly useful for critique, so I'm going to focus on the second and third photos. Look at both of them, and remind yourself that your crotch is supposed to be over the middle of your saddle, not over the pommel.
I beleive part of this is from the pivoting on the knee, lower leg sliding back and sending your body forward, but I really think the root case is that you are "locked" into your two point, and your hip is not closing over the top of the fence and then opening, and that the horse's thrust then acts to push body forward and leg back because your hip isn't doing is job as a shock absorber. It looks to me like you try to bend from the waist to compensate for the lack of motion in your hip, and that creates the roundness in your back.
I would recommend you try two things. First, tie your stirrups to the girth with bits of baling twine to anchor your leg in place underneath, and then do a lot of flat work moving in and out of your two point. You will feel the resistance of the twine anytime your leg slides back, and you'll feel what it's like to have your leg solidly underneath you while you work in two point.
Next, practice the "rider's pushup." Starting at the halt, find a balanced, secure two point, with your crotch over the middle of the saddle and your weight balanced over your stirrups, one that you can maintain without pressing on the neck. Slowly fold your hip, closing your hip angle and touch your chest to the horse's neck, all while looking up between the ears and keeping your balance. Then open your hip and come back up to a normal two point, all without touching the neck. This will be very difficult at first. Make sure you're using your *hip*, not your waist. Have an instructor or friend stand next to you to tell you when you straighten your knee or move your crotch over the pommel or round your back. As you gain stability, do this exercise at the walk and trot too. Alternate doing it with stirrups tied to the girth and without, pay attention to what happens to your lower leg and overall balance without your stirrups tied.
You're trying to create a correct muscle memory by practicing the motion that SHOULD happen naturally with the horse's thrust over the top of the fence.
I also agree that I would prefer to see a slightly more generous release.
One final note, in the third photo, your right foot is in the stirrup backwards - the stirrup leather should lie flat against your calf, with the half twist in the leather towards you, not away.
I apologize if this very detailed critique makes it sound as if you're doing everything wrong; that's not the case at all. At the very worst, you are an able, pleasant, non-interfering rider with a few form faults. The best is much, much better than that. You are the type of rider that I would love to teach and help, because a few small tweaks will improve your riding leaps and bounds.
Good luck, and I would love it if you would post back and show photos of your progress in a few months.
Last edited by maura; 05-10-2012 at 06:44 AM.