I'd say just relaxing will help you a lot. Although you may not feel tense, you look it (I have this same problem). In the first picture, you have a good line from hip to heel and almost a line from elbow to bit (shorter reins will help). If you relax into your heels, then they won't twist out as much. Focus on using more of your inner calf than the back of your calf (unless you do hunters). Also, relax your elbows. They don't always have to be at your sides. Once you get the correct rein length (which could be shorter than in these pictures), you can move your elbows forwards, backwards, higher or lower. This will give your horse a release without throwing away your contact. For your heels, think about relaxing your ankle and melting down through the stirrups. This way, you aren't pushing down into you heels which causes tension in the ankle and sometimes a chairseat.
I think you could benefit from trotting and cantering in jumping position because that really helps get your weight into your heels as well as establishing balance. Other than that, just lots and lots of practice. You've got good basics- looking where you're going and not resting on your horses mouth (I've always thought it's better to have a hand that's too soft than one that's too heavy). Cute horse :) Keep up the good work!