Many of these are yoga positions. Work to achieve them slowly. With time it will come. ~ Muscle assisted shoulder stretches: tricep stretch, one arm over the head, both arms over the head, cow face yoga position, grab an elbow be.
You have a great body but a bad posture and hunched shoulders can portrait a totally different picture of you. Your tummy may look fat and you will look shorter than your actual height. You must correct your body posture to avoid any possible health issue