allRGB is a pretty cool website. It collects (and encourages the creation of) images that contain every 24-bit color exactly once. Why? Well, why not? It's completely pointless, a totally arbitrary constraint based on technical happenstance. That's what makes it delightful. See: Tunnel Man. Art. Lipograms. The pointlessness is the point?
Anyway, there's one kind of image that appears on the site a few times: Photos that have been recolored to fit this website's weird color constraints. Zoomed out, they look pretty much like normal photos. Zoomed in, they look a bit like some pointilist paintings. How the hell?
I think it's dithering. I don't understand exactly how dithering works, but I read parts of a long blog post about dithering in Return of the Obra Dinn, and the basic idea is that you approximate the original image in a way that spreads out the errors.
If true, I find it funny this approach could works. Somehow you can use the same technique to turn a color or grayscale image black-and-white and to transform a color image into a crazy use-every-color-exactly-once palette. It's a wild thought.