You need to work on your leg. When you post your leg comes right off the horse because you're pushing yourself out of the saddle rather than letting your horse's motion push you up. This is partially because your horse doesn't have a lot of movement (not much bounce and swing to his trot) and because he is behind your leg (lacking impulsion). Your arms are also locked, and not allowing for much natural motion at the trot, which might be part of why he isn't trotting out as much as he should be. Because of the tightness in your arms, you're also posting with your hands. You look a little tense up there. As for your release, I think it was adequate for the height you're jumping, and you should continue doing this kind of release (crest release) while you're still learning. Other than that your posture is nice, and I think you'll have quite a nice position with a little more practice, and when your horse is going more forward and you're feeling a little more confident.