Although you're heel is down, you're not using your leg in the most efficient way and it seems like you're just standing in your stirrups instead of going into 2-point. This is also allowing you to arch your back over the fences, when instead you should try to bend at the hip for your fold. It's hard to tell from a picture, but you may be more prone to catching your pony in the mouth when you arch your back, as it seems like you also pull your hands back a bit when you do that. (your reins are in a good place in the first picture and you're back is straight, but in the other pictures when you're noticeably arching your reins seem too tight and I'm guessing you could be catching your pony in the mouth in the other landings?) A solid 2-point should also help with learning how to control not jumping ahead.
And when I say fold over the fence, at 18inches it's more of getting into a quick 2-point if anything. Don't get into the habit of looking like you're jumping 5ft when it's only crossrails.
I agree with the other posts. You could use more angle in your leg. That second to last picture is very telling. Your crotch should stay behind the pommel when jumping. Instead of standing in the stirrups, maybe try shortening your stirrups a hole and practice letting the horse's jumping motion fold you into a proper position over the fence. If you fold more, it should allow your arms to come more foward to give you pony a bigger release. Back is flat and you're looking ahead which are both good.