Not bad, really.
Lower your stirrups a hole or two. They're too short, so your knees are too bent and you're sitting too far back on the saddle and too far back on your pockets.
Your saddle may be too small for you as well - but lengthening the stirrups may be enough.
Ideally, one would want to have a straight vertical line run through your ears, down your back, through your hip, and then down through your heel. In your picture, you can draw a vertical line and see what's out of place quite easily.
With longer stirrups, it will be easier to get in this proper position. Right now your feet are too far in front of you and your butt is too far behind you. Chair seat style. With longer stirrups, your weight will be less on your rear and more on your upper thighs, bringing your rear end forward and allowing your legs to easily move back into the correct position.
Another thing you can try to find the correct leg position: stand up in your stirrups and get yourself balanced, just as if you were standing on the ground. Then sit down without moving your feet. This is the correct position, with your feet positioned to support your weight without moving. Try standing, getting balance, and sitting several times, till you get a feel for the correct position. Then try it at the walk. Remember you can always stand, check your position, and reseat yourself if you feel like you're getting out of whack.
Being open to constructive criticism is the best thing you can do - I applaud your for it!
Good Luck.
Edit: Here's a picture showing the vertical line I'm referring to -
http://www.google.com/imgres?hl=en&client=firefox-a&hs=HcW&sa=X&tbo=d&rls=org.mozilla:en-US:official&channel=fflb&biw=1025&bih=498&tbm=isch&tbnid=CURRvOrwVu3l5M:&imgrefurl=http://regaljada.tripod.com/saddleseatequitation/id21.html&docid=77oACbSPyGySUM&imgurl=http://regaljada.tripod.com/sitebuildercontent/sitebuilderpictures/goodposition.jpg&w=220&h=303&ei=ZVe1UPK9FpSI8QT7-oA4&zoom=1&iact=hc&vpx=168&vpy=143&dur=1306&hovh=242&hovw=176&tx=86&ty=153&sig=117447958458196474534&page=1&tbnh=161&tbnw=117&start=0&ndsp=10&ved=1t:429,r:6,s:0,i:105
That pic is for English, but it's the same as Western, like this pic:
http://www.google.com/imgres?hl=en&client=firefox-a&hs=BJr&tbo=d&rls=org.mozilla:en-US:official&channel=fflb&biw=1025&bih=498&tbm=isch&tbnid=NRE4bpAp-s1mmM:&imgrefurl=http://www.gaitedhorses.net/Articles/Equitation/GHEquitation.shtml&docid=znZVQ7itS3Np6M&imgurl=http://www.gaitedhorses.net/Articles/Equitation/images/straightwestern2.jpg&w=300&h=243&ei=yFe1UIbTBouk8AT2-4Ag&zoom=1&iact=hc&vpx=129&vpy=132&dur=3511&hovh=194&hovw=240&tx=148&ty=105&sig=117447958458196474534&page=7&tbnh=138&tbnw=190&start=93&ndsp=15&ved=1t:429,r:99,s:0,i:384
And of course, this position is for the walk; when the horse is moving at different gaits you want your upper body to move a little, but your legs and seat would stay the same.