I can't really see that much from the photo, but I do think you need to have your upper body back more. I assume your into dressage and jumping from your saddle? Your heels could possibly go down a bit more, but don't force them. I think you are looking pretty good otherwise. =]
You kind of sit the same way I have a tendency to sit, so I'll just repeat what my instructor says: Sit centered on your seat bones, both lower back and belly pushed outward, shoulders up and back. Whenever I hollow my lower back, my pelvis tips forward and my leg shifts further back, and I appear "stiff". If you can manage sitting without rolling forward, you'll have an easier time getting that shoulder, elbow, heel line.