One of the immediate things that stands out in these pictures are your heels. Keep them down! If they're not down, you're less likely to remain in the saddle (I've learned from experience over the past 9 years). It'll also help you with your balance.
Additionally, don't stand up that far in the saddle. In other words, keep your butt closer to the saddle. There shouldn't be that much empty space between your butt and the saddle. When you're standing up like that, you also want to bend at the waist, stretching your arms forward to release your horse's mouth. You're just almost standing straight up, instead of leaning forward.
Also, stay in the saddle for nearly twice the amount of time that you are currently. You should be in two-point position until your horse's hind legs touch the ground. In several of the pictures, you had already regained your seat while your horse was in midair.
I would also work on tightening your leg. Majorly. It appears to have slid quite a bit from these pictures.
Lastly, due to your lack of jumping experience and your form over these jumps, you should definitely not be jumping that high. It's putting both you and the horse in danger if you are so inexperienced with regards to jumping. Try something small, or even trotting over ground poles.