First of all let me say that I like that your are relaxed and that you have your weight sunk down into your heel. A couple of people have noted that your posture is a little slouchy an crooked. I agree you've got a little bit of a chair seat going on. You want to sit up nice and tall and get your leg back underneath you a little bit more (just make sure not so much that it gets behind). Ideally you want a nice straight line from your shoulder to your hip to your heel. As far as the crooked goes, it's next to impossible to sit straight in the saddle with your free hand down by your side. If you pick up your free hand and hold it as if you were riding with two hands you'll generally end up sitting a lot more squarely in the saddle (this is assuming you're in a horsemanship/equitation class. You'll want to leave it down for western pleasure). Actually the lady in the picture behind you is doing it, although preferably your hands will be even. Other than that look up, smile and invest in a pair of black pants. You can find some at Walmart for less than 10 bucks and it will give you a much more polished look. Also, I would suggest tucking in your shirt for horsemanship.
OK that's all. Except I have to say you and your horse make a beautiful pair, and you are totally not fat! Good luck :)