Ahead of the motion over jumps, just wait a little bit longer. All you really need to do is tip forward a little and it will bring you more than enough outta your seat to make those sized jumps. Also turn you toes in :)
Just let the horse carry you over the jump, instead of you jumping for the horse. I just thought I would tell you that, because I always jumped ahead, then someone said it like that I all of a suddenly I didnt jump ahead anymore while I kept that in mind. :)
You're really riding on your crotch which is caused by tipping your pelvis quite far forward. This is really evident in pic #5. When you tip your pelvis like that, it makes you grip with your knees which is why your legs are slipping so far back when you jump. Once you find the right position with your pelvis, your horse will also be less on the forehand. The last two pictures really illustrate how heavy they are on the forehand. Try sitting at the halt and rocking your pelvis so that you're arched way forward and then way back. When you feel your hips loosen and your weight evenly on your seatbones, you'll know you're in the right place. Good luck!
Feel free to tear apart my eq and tell me everything I'm doing wrong/right. =X If I had to critique myself, I'd say leg forward, hands forward more and stay back behind the pommel. X] Thank you! (:
You are gripping with your knees too much. This causes the lower leg to lose its contact and fall too far back. This causes your upper body to become "topheavy" and it tips forward. As a result, your hands have to brace on the horse's neck to keep from falling forward. You back is too arched. You need to flatten it a bit. The too arched back causes your whole body to stiffen.
The same here. Your lower leg it breaking outward because you are grabbing with the back of your calf. You need to let go with your knees, point your toes more forward and hold the horse with the INSIDE of you calf. Relax the knees so that they can flex and allow your seat to shift back behind the pommel, where it belongs 
Lower leg needs to come to the girth. It has slipped too far back. Not part of the question, but the martingale is a bit too snug, IMO. 
You need to hold the horse with the inside of your calf to allow your lower leg to shift forward and keep the toes in. 
You have a lot of talent and these problems are easy to fix, with proper coaching. It is good that you are able to listen to all of this constructive help.