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I recently spent some time developing a compositor/renderer for a 3 color (black/white/red) e-paper display with very low resolution. I ended up learning a lot about dithering and how to give it that nice old-school dithered aesthetic.
>lower the resolution of your pictures considerably (with interpolation), do your work on them, then re-upscale them without interpolation.
most images are too high-res for any dithering to be noticeable on a decent display. if you dither a high-res picture with too many colors, it just looks like you added a very light noise to it rather than actually dithering it.
>use a very limited color palette.
if you use too many colors while you 'dither' an image it's hardly noticeable. this parameter goes hand in hand with resolution; the lower the resolution you have the more colors you can get away with and retain a dithered look.
>for an old-school graphics aesthetic; use a custom palette for your image
if you look at old-school graphics, they picked palettes that minimized the amount of dithering they had to actually do. they would pick colors that allowed them to have large contiguous region of one, undithered color.
>pre-process your image by bumping up the contrast
ideally, you want to have the darkest regions of your image be undithered, contiguous blocks of your darkest indexed color, and the brightest regions be blocks of your lightest color. essentially, you want to fit the dynamic range of the original color space into the new faux-color-space given by dithering
>pre-process your image by posterizing it
essentially, if you manage the color space resolution first, it helps retain feature edges once you switch to the dithered color space, which tend to become very undefined and hard to distinguish between in dithered images.
