I can't really tell from the picture, but it is possible that you are letting your right shoulder collapse in a little bit. You will have better form and will gain a fraction in draw length if you consciously push with your bow arm and keep your shoulder from collapsing in. Keep some bend in your bow elbow though, as you seem to be doing. Are your string fingers relaxed? It seems like there may be some bend in your hand at the knuckles. If you're pulling with your back muscles, you should be able to relax everything from your elbow down, so that the back of your hand flattens out, i.e. just acts as a link between your elbow and your fingertips.
Again, you may already be doing these things, but it is hard for me to tell from the photo.