First of all, find a good site for stocks. Try planet renders, deviant art, photobucket, etc.
Next, when you use a stock, you want to transform it not slightly edit it.
You used a premade piece of art basically, you want to take something that hasn't been edited heavily and then go from there.
After that, you want to make sure when you do the sig, the focal point is on the person, thing, w/e
Next too far off to to the side. Also you tried to use a bit of smudging, smudging should be subtle, and elegant.
Use small brushes with different settings for different out comes.
And lighting should always be natural and flowing. Though this piece has none, you should remember that.