everything you need to know is right here.
Originally posted by Bashar Teg
selectively drop expendable colors from the pallet and find a balance between that and dithering
also try different types of dithering and (sorry to insult you by repeating it, tenderbuns): find the absolute largest starting point you can find and zoom in close to make sure that it's genuine hi-res, and not originating from those jerkoff sites who recompress images to a larger size.