1: Starting at the base. You have a nice heel on the flat, but it seems like you are bracing in your stirrups. Your leg has been shoved forward. You asked for leg strengthing exercises, so you know your leg isn't that strong. Lots of no stirrup work, just take the stirrups off your saddle. Once you build up your leg you will be able to keep your calf on much easier, but you still need to try to keep your leg in the correct spot to build the muscles correctly. You leg needs to under your hip, not in front of the girth. From the picture it looks like you might be pinching with your knee, but I'm not sure because of the knee rolls on your saddle. You look like you have really nice light soft hands which is great. Just try to keep your hands even, and don't roll your right hand up like that. Look up to where you are going. Chin up!
2: Now you can see where you weak leg really comes into play. You obviously know what you are doing, but just seem out of practice and weak. You are really bracing against your leg, but I would rather see this than your leg swinging back. I like how you are looking up to where you are going. Your shoulders should never get in front of your hands. You don't need a huge release, and yo should keep light contact, but you shouldn't get ahead of your hands.
3:Look like an awkward fence for both you and her. You seem like you got popped a little, and your leg came up, and you lost your seat and your balance.
4: DON"T LOOK DOWN AT THE FENCE!!!! Looking down at the fence, you will land there, and it hurts to land on the fence. Trust me on that one. lol. Shoulders back and sit up tall.
I think you and your horse look good, you just need to build muscle and practice some more and you guys will look stellar.