Ok, the mane things I'ld say are.... Sit up straight!!! You seem to sit very 'hunch' keep you back straight and pull your shoulders back a little. Heels down and lift your hands a little, you look like you rest your hands on his neck quite a bit. Try not to turn your toes out when your over the fence. Maybe shorten you stirups a little more for jumping. Have you got any videos? It's hard to give advice on pics alone.
i saw that in the other forum it said that you've only been together for a month or so, your working great together for that short a time!
I would maybe suggest that on the flat you might want to lengthen your stirrups a little, but that may just be me (I always seem to have very long stirrups). That could help you to lengthen your legs a little, bringing them underneath you more which in turn could make your top half straighter.
as above though, just keep your top half more upright by bringing your shoulders back and imagine pushing your chest out more.