The hair outlined part (that green stuff) is a bit distracting and kind of lowers the quality down slightly (it
doesn't really look too high quality, nor does it look "intentional" in the respect it's serving a
purpose). The scan is also shifted a bit too far down.
You can also consider shifting the scan a bit to the left (but not centre) so that you can overcome the empty
problem.
Hmmm. The arrows also have a different quality to the scan you have used. Perhaps you can make them more solid like the
characters? This wallpaper will do with some good geometry (like shapes and stuff), even textures will do (even on
shapes) behind the Samurai C. gang.
Just a bit of that and a bit more work will be fine and can be submitted. You don't have to fill in the whole bg,
just make sure you've completed your concept a bit more, because it seems a bit raw at the moment ^_^ Nice work!