Your back is at the edge of beinging over arched in the pic. It could be flatter, but it isn't seriously flawed. So, when you say that with stirrups you arch more, that is a little concerning. I agree about the knees and the lower leg - you definitely need to get your lower leg more under control and focus less on the back for a bit. YOu also look like you might have some piano hands, or at the very least, you have a pretty serious break in your wrist. That may be because with your legs gone and your back arched, you are supporting yourself on your hands.
I used to do hunters, but with my instructor, backs were expected to be straight. I spent many a lesson riding with a crop down my back. Now I'm doing jumpers, and my trainer's trainer (who has done grand puissance and trains grand prix riders and horses, has us actually slump a little bit going up to the jump. Once you get the feeling, this is incredible at helping you stop jumping ahead, because you're all ready when the jump comes, so you don't have to make a big move to be in position. It also allows you to get out of the horses way faster, so that they can really snap their legs and back up. Still though, when in the air, the back is straight, not rounded or arched.