Todd's website has a nice flash illustration which shows how to convert your scratched black n' white scans (bitmaps) into scalable color pictures (in vector format). Though it designed for the Flash software, the trick is unversal and can be applied in all software including Gimp - the poor man's photoshop.
You'll need the macromedia Flash Plug-in to view most of the stuff on this site.
See the tutorial here.
