Hi there :) Your heels seem to be nicely down and you appear to be placing your weight on your buttocks which is good! However, in all your shots notice you are leaning somewhat forward.
Here is a trick you can try to remedy it which actually works VERY WELL.
While riding, simply push out your belly, so that you are pointing your belly button towards your horses ears. I know it sounds ridiculous, but its an exellent way to prevent yourself from leaning forward. Doing this hollows your back and immediatly brings your shoulders back (try it now infront of the pc and you'll see what I mean) doing this has been helping me as well since I tend to lean slightly forward when I ride!