Congrats on you're first jumper show! I think you have a great base, your leg and seat look good. If I was to guess in that first picture I would say your horse got strong to the jump and to compensate you thought you would get your contact back early and sit down aswell so he didnt run on the other side of the fence. Lots of flat work will fix that, working on lengthening and shortening the stride with maintaining the same pace and a soft mouth. Another suggestion I have for you, if your going to continue doing the jumpers try out a running martingale instead of the standing martingale.
And remember thumbs up and heels down ;)
~If horseback riding was easy they would call it football.~