Likes: Nice heel/hip/should er alignment. Nice lower leg/foot postiion.
Things you can improve: Hands more upright (you know this) ,Thumbs don't need to be straight up, but at least match the angle of the shoulder. The small of your back is swayed in a bit. See if you can straighten up - that will save your back and bring your seat-bones into better contact with the saddle. You shouldn't have a loose rein, but rather have light contact whit the bit at all times.
Have contact with the mouth at all times, even a light contact if the pony has a soft mouth. A good bit of what has been said, but if you want help on keeping thumbs on top, put a whip between your two hands on the reins, just under thumbs to keep them on top, also to keep hands symmetrical when turning, while you're doing this, imagine giving your pony a big thumbs-up!:) Wrong diagonal in pic 3 and 4. Push the lower leg forward a little bit, but you shouldn't see your toes if you look down. In pic 6, are your hands underneath you a bit? Maybe it's just the photo angle:). Otherwise, lovely seat:)