Notice how this riders right shoulder is lower than her left. This will impact her horse's ability to do many of the maneuvers we ask of our horse. For example, there is a good chance the horse will not be able to pick up the correct lead in a canter, since the weight of the rider is not balanced in her own body. I recommend two exercises to help you determine your own body positions - sit on your saddle in front of a mirror. See what your body looks like at rest, on an innate object. Then, video tape yourself on your horse. You would be amazed at what you see!
Another interesting article on the topic Rider and Horse Asymmetry Article - how this affects your saddle and tack: