First things first, take all of JR's advice then maybe supplement with mine. He knows more than I do. Check out this thread (http://www.discgolfreview.com/forums/viewtopic.php?f=2&t=21385
) and some of the diagrams illustrated. Notice how the disc is pulled through on a straight line from reach back to the hit. You are rounding the whole time, making more of an arc with the disc rather than a straight line. Here is a good little gif that shows what path the disc should take:
Note the shoulder position and how they face back towards the reach back then square when the disc is pulled to the right peck, then and only then do they start to open more. Your feet and posture looks pretty good.