In the first jumping picture:
You're gripping with your knees, and consequentially, your lower leg has slid back. I think doing some stirrupless work over small grids and such would get you using your whole leg, instead of bracing with your knee, and relying on your stirrups to support your lower leg through the jump.
Your release is pretty good, but if you're trying to do a short crest, you need to lower your hands on her neck bit, they're a little too high, but not bad at all, :]
Also, make sure you work on staying out of the saddle after the jump, it kinda look like you either got left behind or just didn't get up enough.
You're looking up, which is awesome =D
Your back and shoulders also look very nice. There's not roaching or hollowing.. which again, awesomeness XD
LAP HANDS. Lol. Get those hands on front of you! :]
You also seem to have a bit of a death grip on the reins too, so work on a softer hand as well.
Again, you're gripping too much with your knee, and not enough with your lower leg, so really try and stretch you heel down, and wrap your legs around the horses barrel. Not as in grabbing hold for balance and security, just a nice quiet leg is always good.
Like you mentioned with your arms, they're a little straight, but that can be fixed, and I'm sure you've been working on it :]
You're in your horses mouth way too much, she's got her mouth open and gaped, and I can see your hands are tight on the reins.
She seems a little tight over the topline in this picture, and has fallen onto the forehand. She's probably done this because you'r in her mouth, and off her sides with her lower leg. So REALLY work on loosening up the reins, and pushing her forward.
She's veryyy tight through her front end, probaby because you're bracing in her mouth.
I LOVE your heel here, it's very very nice! Kudos =D
Lengthen your stirrups ATLEAST one hole. For just being in a half seat at the canter, you're waaaay too far out of the saddle.
You've begun doing what I did, just standing on the stirrups and pinching with my knee. If she were to spook or swerve to the side for some reason, you'd be off.
I think for your half seat, you should practice it without stirrups, because you rely on them wayyy too much for your half seat to be correct and effective.
Your reins aren't as tight in this photo, but you're still kindof bracing in her mouth. Also, when you rise into half seat, let your hands follow you into a higher postion. Your hand to mouth connection is severely broken.
A half seat doesn't nessicarily mean that you get out of the saddle with your seat, it means to shift your weight to the front of your pelvis and close your hip angle in preparation for a jump.
This is what a half seat SHOULD look like:
Better with your hands! They're up where they're supposed to be, but you still have an overly tight hold on the reins, especially if you just came from a jump.
Again, you're pinching with your knees, and standing in the stirrups. You've gotten into the habit of a false half seat.
From what I can tell, you rely way too much on the tack [specifically reins and stirrups] which probably also means your balance could be better. I really really strongly think you would benenfit from a lot of stirrupless riding, and riding on a lunge line without reins, so that your seat can be completely independent of the tack and aids. I honestly think you should polish yourself on the flat before you do much more jumping. You have a lottt of potential, and your horse is an a+ mover. I love the hind engagement that you get out of her, and she seems to be trying to round, but your tight rein hold does get in her way. She's vrey flashy, and I'm in love with her knee action. I think you should consider doing 3 day with her someday, because her movement seems very typey, and could get you some very nice scores in the dressage ring, with some polishing.
I would work a LOT more on pole work. Put like... a gajillion trot poles, and just work her over it. Practice lifting up her shoulders and really pushing her forward with your lower leg, because she's forehandy, and getting her a bit more on her rump, would give her a world of benefits on a jump course, or just over a set of poles.
From doing all of this, I assure you, you'll both become better athletes, and more competitive in your sport.
Hope I helped :]