the third shot is the best, and the one of you at your lesson, which is very good.
You have a very good way of connecting your upper body to the saddle and your seat bones. I can see that you are right on them, and when you sit up, I see the potential for a very good rider.
Yes, you need to roll your forearm over to get those thumbs on top (and by the way the reason for thumbs on top is exactly that; to cause the forearm to roll over, whcih brings your elbow into correct alignment and close to your body where it belongs) And that was my next thing. You need to bring your elbow in and closer to the body. You will need to take enough contact that the horse gives a little bit in the jaw and poll and then ask him to step up and under with his hind end. This might sound vague. It's hard to find concrete , specific ways of putting something that is a matter of feel into words.
On the black your horse leg is draping down much better and not so turned. On your horse, it looks like you might be gripping with the back of your calf. Maybe that's a hunter thing, but not in dressage. You will want to work on keeping your legs open and letting them fall down ward all the way to the ankle
Try riding without stirrups for a bit, then just lifting your toe and popping your forefoot into the stirrup.
You're posting higher than necesary but it's not the end of the world.
Look at those pictures; the third one at the canter, and you on the black. Can you feel now how it felt then to be in that position? Those are the best of the ones you offer. Seek toward them and beyond.