It looks like your saddle is pushing you into a chair seat, or your leg is just unsteady. This is the biggest thing I see, otherwise your upper body posture is correct, but your arms could use some work.
I would really like to see you working to keep your legs under you and really put them on, riding without stirrups and practicing a lot of forward sitting trot with a long leg that is underneath you. When your legs are stronger, I would like to see your stirrups 1-2 holes longer, just so the stirrups don't put you in such a chair seat.
I would also get your saddle fit checked, just as a hunch I think that it needs more flocking in the back to get the low point further forward, and make the saddle so you can sit above your leg, not behind it. If you're really strapped for cash, Cashel makes a good riser pad.
As for your arms, you really really really need to get your elbows IN. The less stable your elbows are, the less stable your hands are and the more apt to pull you will be. The more we pull, the more the horse objects, and if becomes harder to ride, so we get more unbalanced and pull harder etc etc.. Long story short, put your elbows on your hips and leave them there!!! (Except in a release, then they follow the horse's mouth) Also, you need to put your thumbs on top and bring your hands down, right near the withers. The easiest way to correct this is to turn your wrists so that your thumbs are pointing forward and slightly down like this
http://imagebank.ipcmedia.com/imageB...d1f4d98dcc.jpg. Your hands should be a little bit lower though.
A note on form - I've won hunter o/f classes without my bum leaving the tack, except over fences. The best way to ride a course is in a true half seat and not a two point. This way we can balance our horses better without making huge looking corrections.
And a note on turn out - your stirrup leathers are much too long and should be trimmed so they don't extend past the saddle flap. Your full cheek needs a keeper so it doesn't get stuck on things.
http://lizditz.typepad.com/photos/ho...ewebalbum.jpgp Also a half pad looks a bit more polished, this is only a matter of taste though.
Good job! Keep practicing!