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Continue to ChatQuery : If it's not a photorealistic image, and/or has limited color palette, Photoshop Save For Web in PNG-8 format could be your best option. Here you can specify the number of colors (2-256), the https://vmapp.org/query/20237/If-It-S-Not-A-Photorealistic-Image-And-Or-Has-Limited-Color-Palette-Photoshop-Save-For-Web-In-PNG-8-Format-Could-Be-Your-Best-Option-Here-You-Can-Specify-The-Number-Of-Colors-2-256-The/