Bear in mind that I'm not a qualified coach.
However, having looked at your pics, I think one of the obvious problems you can address immediately is your posture. Your back is extremely arched and your chest is sticking right out. This will stop you getting a good line and prevent your back muscles taking the load.
You need to think about keeping your body straight rather than putting a big arch into it. I suggest you get a light bow or exercise band and practice better posture. Before and during the shot you should think about tensing your stomach muscles a little, lowering the rib cage (rather than sticking it out) and perhaps even leaning forwards ever so slightly, putting a little more of your weight on your toes rather than your heels.
More importantly, you need to find a recommended coach in your area who can help you with stuff like this. Analysis of pictures over the net can be very useful, but there's no substitute for competent face-to-face coaching.
Good luck and good shooting
