First off, I love riding the older, more experienced horses. They can teach you a lot because they've pretty much done everything.
Your back seems to be nice and flat, except for in the last photo where your back is a little hunched (or maybe it's the baggy jacket?). Your toe could be turned in a bit more and you seem to be laying on the neck a bit. Your leg is where it's supposed to be in the second picture so I'm pretty sure you have a solid leg position, just get your toes in more.