Well I think you are off to a great start by having the opportunity to ride multiple different horses that are all a little different in how they move and respond to you. While as a beginner its helpful to ride only one horse for a while to develop your position, riding different horses really helps you to become an adaptable rider and teaches you to really read and listen to a horse.
On to the pictures...
Photo 1. The first thing I notice is your upper body. You seem tense and you are leaning in with your left shoulder. Bring that shoulder back. I do like that you are sitting up tall. I hope you are getting ready to post to the trot because your bum is quite a bit out of the saddle to be sitting to the trot. I'm not sure what you're doing with your right hand but your left hand is doing a good job of keeping your thumbs up on top. You don't want "piano hands" because they aren't effective when cueing the horse through the reins. Your foot is in a very faulty position. You need to slide your foot out of the iron until its on the ball of your foot. Then position the iron so that way the outside bar is against your pinky toe and the inside bar against the ball of your foot. This will help your ankle to flex more easily and can help keep your toes pointed more forward. I love how you are looking up and where you are going!
Photo 2. Please tell me you are not wearing sneakers!? That is very dangerous because your foot could slide through the stirrup because there's no heel or the wide tread on those shoes can get your foot stuck. This is especially dangerous for you because you have not established the correct heel position! Put on those paddock boots you had in the first picture. :) I like that your leg is back and although you may have just been cueing your horse, this is a good position. Piano hands! Get those thumbs up. That helps you to cue your horse through your fingers and not have to pull back with your entire arm. Now this might be because your shirt is baggy but it looks like you're slouching a bit. Sit up and be proud that you get the chance to ride a horse at your young age. Again, good job of looking up. :)
Photo 3. Not much to tell from this picture but your piano hands! ;)
Photo 4. Riding bareback is a fun time to just have fun. You get the opportunity to sit close to the horse and feel how he moves. If you want to still practice your position, here I would have you get your toes up and practice keeping your heels down. Good hand position here but now you need to look up!
I hope that helps you out!