I think the problem with PNGs are, that they embed gamma, so depending on the device you look at those PNGs (Mac, Windows, iLiad, ...) the colors/greyshades are a bit shifted, for a more detailed description see:
Gamma Problem in PNGs.
So PNGs a bit "over-engineered" and not very well suited for these kind of things, as the colors are a bit different on each platform...
So the best would be to either use gifs (and make sure, that the drawing application does not alter the color information) or simply use a PDF
