You look nice! I understand in the first picture you're probably trying to get your horse moving (is he slow?) so your leg is farther back than usual. Next time, try sitting up taller - You're not necessarily slouching (well, maybe, but at least you have a nicely postured slouch!) but making your upper body taller will get you noticed.
Your posture looks good for a jump of that height, all you need is practice, really. Seeing your leg again here, maybe you do ride with your leg back a little. I'm not sure if he's just a really lazy or slow horse, but try to keep your leg a little more forward. Try carrying a crop with you over fences so you don't have to lose the support of your leg when you're trying to kick him over fences.
OH! And try keeping your shoulders back - Practice "over doing it" - You might feel silly, but you really don't.