You tend to ride in a chair position. It's not too bad in the second picture, in the rest of them your leg is too far forward. When you're in the correct english seat, you should be able to drawa a line from your ear through your shoulder, then through your hip to your heel. A good way to test this is to stand straight up in the saddle, then sit back down without moving your leg. It takes awhile to get used to, but your balance and center of gravity will be much better for it.
I couldn't see your hands in most of the pictures, but in the 3rd picture, they need some fixing! You need to close your fingers, with thumbs to the sky.
Does your horse pull on the bit? Your elbows look locked and they should be more to your side. Also your shoulders are hunched. A good exercise to fix both those things is to ride with a yardstick or a crop behind your back through your elbows.
Lastly, look up! You always want to look where you're going and constantly looking down will only confuse your horse.