You're a bit far out of the saddle, so your leg has slipped.. you probably know this :)
The 2nd to last picture looks better than the rest... You're more over the horse's center of gravity and your leg looks better too.
In the 7th pic (and maybe some others) your heel came up a bit. Just really sink your weight in your heels. Actually, the jumping ahead probably causes this problem. So also, your leg wont slip as much if you're more down in your heel.
It's allllll relative ;)
BTW: OMG CUTE HORSEY! :)
But seriously, she looks darling!
ETA: Arg, Ray! You beat me to it!
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman