It's so... it's so...
Tiny.
(Sorry, I recently got a nice screen resolution, and I'm still rubbing it in people's faces).
In any event. You wanted feedback, and boy, oh, boy, are you going to get feedback.
You did a pretty decent job on making the stock photo not look so photo... ish. Unfortunately, it's still very
obviously of a style very different from the scan that the scan really stands out, making it look really 2D against the
background. The main problem there is the edges; and you've already noticed the worst problem with that (that
obnoxious light source at the back of the girl's head).
Honestly, you really need to have a fair bit of experience to deal with the problem you're running in to. Even when
I'd been walling for some two and a half years and I was trying to pull off a similar trick (making my scan fit
into my background for it), I had to pull a lot of the stops out to get it satisfactory; and the reward for what
you're trying to do here isn't going to be nearly as visually stunning as the amount of work that you're
going to have to put into it.
So I'd actually recommend going more for style than realism. Simplify things. Reduce the background down to a
pastel dreamscape sort of thing