In the first picture, you are pinching with your knee. Pinching with your knee tips your entire body forward and your lower leg back. See how far back your heel has come? The moment you pinch with your knee, all your weight it sent forward to your arms. You have lost your base of support and you are relying on your arms to keep you up in your jumping position. Do you see how your arms are resting on your horse's neck? That's the only thing supporting you. Not only does this throw your horse off balance over the jump and through the landing, it is also dangerous for you. If your horse were to stumble on the landing, you would be pitched over his shoulder.
The fix? Start by building up your base of support. I'd go back to smaller jumps until you have established strong legs and a safe, balanced jumping position. Strengthen those legs through lots and lots of 2-point at all 3 gaits. When you're in your 2 point, think about sinking your weight into your heels and remaining balanced over your horse. You should be able to hold the 2 point position at all gaits with little or no help from your hands. Once you have established a balanced 2 point position at all 3 gaits, start applying that to your jumping. Work over little jumps first and concentrate on keeping your lower leg still. Your lower leg should barely move as you jump. When your base of support is strong, you'll be much more balanced.
Establishing a strong base of support will help your form drastically, but there are some other things to keep in mind. In the picture you showed, you are jumping ahead of your horse, flinging your weight forward before he does. Back up that upper body! Right now your bum's reaching for the sky and you're kissing his neck. This is because your are dropping your upper body into your arms. See how your upper arm is aligned with the horse's neck? Concentrate on bringing your whole upper body up and back.
Hope this helps. :)
Every ride, good or bad, teaches you something new.