Your leg is slightly too far back. It can be easily fixed, by waiting with your body and not jumping ahead. Think of waiting for your horse and not moving on take off. You also look like you are gripping with your calf, instead of having your weight equally distributed down your whole leg. Im thinking that your stirrups look just a hair too long. I would shorten them and see if it helps your leg.
Over all you and your horse look really good ! You look like a very compatible pair =]