1. In the first few pictures you are putting too much weight on her forehand, which, in turn makes it much more difficult for her to get it up and over the jump. Lift your shoulders up and back and make sure you're looking up. Your stirrups are also much too long for jumping. Try sticking those up two or three holes.
2. You're pinching with your knees. Your long stirrups are definitely not helping this, but once you put them up, really concentrate on sinking all your weight down into your heels.
3. Your body is too far forward, which is partially caused by your knee pinching, but I'm also noticing it over the trotting poles. When you two-point, you want to think about, yes, picking your butt up, but also closing your hip angles so that your butt is still over the center of the saddle.
4. Your release isn't bad for a beginner, but I would recommend using a crest release for now. Slide your hands up the neck and lay them on her crest, even hold some mane to help you. You also seem to be breaking at the wrist, which is something I've seen often, though moreso in dressage than jumping. Make sure you keep those wrists straight! If it really becomes a problem, you can start riding a bit in wrist braces until you develop enough muscle memory that you stop bending them.
Overall you look great for this being your first jumping lesson, though I would recommend sticking to crossrails until you really get the hang of it. You and your pony look great together, and I don't think you're too big. Just watch that you're not jumping her too often. Once a week is really sufficient... maybe twice. And most of all, have fun! :]