Drag in any image and watch it dissolve into Floyd–Steinberg dithering — error diffused pixel by pixel into a handful of colors. Push the pixel size, swap palettes, then pull the result down as a PNG. No upload, nothing leaves your machine.
A quick aside: dithering is the old trick of faking colors you don't have. When a screen could only show two or sixteen tones, dithering scattered the available dots so your eye blended them into something richer. Floyd–Steinberg does it by taking the error from rounding each pixel and pushing that leftover into its neighbors — so the mistakes never pile up, they spread out. Constraint as a feature. That's the whole lab in one idea.