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The first step is trying to aquire a good scan...
the followng articles may be useful for that:
Let's build a TMA
http://www.scantips.com/basics12e.html
What about film scanners?
http://www.scantips.com/basics13.html
Once you have the scans....most image editing software has a Negative
option
that will reverse the image. For example...
in IrfanView you would open the image
and go to...Image / Negative.
Download IrfanView
http://www.tucows.com/preview/194967.html
(get the plug-ins too)
And, in FastStone Image Viewer you would
select the image and go to...Edit / Negative.
FastStone Image Viewer
http://www.faststone.org/FSViewerDetail.htm