I think you actually have a very good posture for dressage, besides the stirrups. They are WAY too short. Lengthen them maybe 3 holes and you will feel a huge difference. You will be more secure in the saddle.
Then your arms look a little tense. Try relaxing your upper body and bring your hands a little closer together in the fourth picture. Another thing is you must never get chicken arms where your elbows move out. Keep them next to your body.
And then most important, heels down. That will help loads with your posture and security in the saddle.
Hope that all helps, and good luck. = )