Bing your shoulders back and square, this'll automatically help with your arms. Your leg position is good, but try resting your feet flatter along his sides instead of digging in with your heels (I know you're working on it, good for you). Doing so will make your lower leg more effective as a whole because your lower leg contact will be better. (If you keep your legs processioned as such you will likely have sore legs at the end of yore next ride.) Also your saddle has short flaps and the way your leg fits it is nice, but the length your stirrups are at is ideal for jumping. Your leg probably won't sit as ideally on the roll, but dropping your stirrups at least one hole, preferably two would be ideal for flatwork.
I like your lower back and your position in the saddle, you aren't leaning and you are looking where you need yo go.