I agree with your stirrups being a bit short. If this is the length you feel comfortable riding at, this may be indicitive of a leg that needs to do no-stirrup work, which will put your leg in a more natural position, stretching it right down into a more appropriate length. Your leg looks great as short as it is here, so it should drop down into a perfect position. This will also make it easier for you to sit up straight with your shoulders back, as will keeping your eyes up.
As for your hands, I may be completely due to the small size of the photo, but it looks like your riding with your palms down. If this is the case, turn those thumbs up! This position allows you to use your wrists effectively, and in turn your reins effectively.
My only other critique on your position is to look up and loosen up those elbows. They're in a very nice spot along your body, but they need some flexibility to interact peacefully with your horses mouth.
Your horse isn't being worked into a frame properly, it looks like this has been happening for awhile, and my guess is you're not the one that originally trained him into this. He's behind the bit, his hind end isn't engaged, and he is stiff, especially through his shoulder and front end. If you want to become a more effective rider, which you look more than capable of doing, you must remember that just because a horse looks round, doesn't mean he is working well or even comfortably. Sit for a few moments with your shoulders scrunched up into your ears and your chin tucked into your neck. It's incredibly uncomfortable. Now imagine that you had to exercise in this position. This is essentially what you're asking your horse to do in this photo. For a horse to be productively round, he must first be able to move around the ring relaxed. He must walk relaxed before he trots, must trot relaxed before he canters, etc. When you have a happy, relaxed horse, that's when you ask him to move forward. When you have a happy, relaxed horse that's moving forward, that's when you ask him to bend. Gently nose in, gently nose out, etc. When you have a happily bending horse, that's when you ask him to really engage his hind end. It's at this point, and this point only that you can begin to ask him for round movement. Now, he may out of habit suck back behind the bit at any point during this process, but this shouldn't be encouraged. He should be ridden forward with a relaxed hand, seat and leg out of this position if this happens. This process doesn't have to be completed all in one ride, and probably shouldn't. This looks like a horse that's accostomed to being ridden all bunched up, and it may take a week of just walk and trot to even begin to relax him. But that's okay! It's a common misconception that a horse is working well if he looks round. But in the end you'll have a happier horse who moves a million times better and who will stay sound longer. I mean, just imagine how badly you'd hurt yourself if you had to workout every day in the position I described earlier.
I hope I've been of some help, I'm new to this board and I don't know how much criticism is too much :roll: ! Let me know if I went overboard, I was just excited about what I saw in this photo.