A bit of an odd and possibly trivial question...
My father, working in the military photo trade, often preserved copies of precious documents by making photostat copies of the originals. Unfortunately; photostats are white on black image copies, rather than the currently accepted black on white image copies. The original artifact from which I made my transcription is the white on black photostat. So, I've been citing it as a "photostatic image copy" and even storing an unreversed scanned image of the same. Is this the correct approach? I'm not sure whether the format would actually impact the validity of the evidence.