Wow, nice idea! I think it's really cute :)
One thing you can do that'll improve the wallpaper greatly is to fix the extraction issues. You can kinda see the
white outlines of Gavroche where you cut him out of the original scan. He's also a little over-filtered. In the
original scan he actually looks pretty detailed and clear but in your wally he seems like he was blurry when enlarged so
you added a filter to make him look better. But the result is that he doesn't look quite clear enough. The filter
is fine but I think if you find a vector of him or ask someone to vector him (not me >.> can't vector)
you'll be able to make him appear clearer.
The background is nice, no suggestions there I guess :) you used the screencap pretty well, just try not to over-filter
that one was well. Your character and BG must have the same amount of filteration (is that even a word?) to make them
appear blended well.
Hope this helped, you're really doing a great job with it! :D Good luck and have fun with it!