First of all, I want to say that I think you and your horse are on your way to doing really great in the jumpers but in order to do that, you really need to go back to the basics of english riding.
The first thing I see is that you are staring at your horse's neck in both pictures and if its in both pictures, you are probably doing it all the time. Look up! When you look up and where you want to go, your upper body sits back, too. Don't worry about your horse; his head and neck aren't going to fall off! ;) You'll be amazed how when you look in advance, you'll be more prepared.
Your back looks okay. I can't really tell from the angles of the photos what its really doing but you may be arching your back just a tad. Relax, you look very tense.
Next are your hands. Okay, first picture looks like you're doing an automatic release and in the second picture, you can definitely tell you are holding your position through his mouth. Ouch! At your level, you do not need to be doing an automatic release. To help hold your position and get off his mouth, grab hold of his mane a quarter of the way up his neck from his withers.
Lastly is your seat, legs and heels. Shorten up your stirrup leathers a hole or two, maybe three. You'll know the appropriate length for jumping by just hanging your legs naturally while on your horse and the stirrup should hit your ankle bone or slightly above. Flatwork can be just a bit lower. Your pictures provide you an example of jumping ahead of your horse's motion. In the second picture, you are getting up in the saddle--ahead of your horse's thrust into the air--and putting all your weight on his forehand. You need to go with your horse, let him do all the work so you are pushing his shoulders down. He needs to be able to lift his shoulders up so he get get his knees up and clear the fence. He'll find it very hard to do when you are putting all your weight up there. :) The first picture then shows the second stage of jumping ahead, where you are now getting behind the motion of his jump. Your butt may or may not be touching the saddle right now, I can't really tell. I'm pretty sure though that in half a second, you'll be down on his back and/or getting smacked in the rear by the saddle. While your horse is in flight, you need to be OFF his back. You also need to have even contact with your thighs, knees and calf. Right now, your just gripping with your knees which is contributing to jumping ahead. Having contact with all three holds your position. Getting your heels down is something to work on, too. Practicing lots of two point and even riding with no stirrups is key right now. Put away the jumps for a while. You really need to get your position down before you move to jumping.
Opportunity is missed by most people because it is dressed in overalls and looks like work. ~Thomas Edison