Tilequant
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SkyTemple comes with a tool (Tilequant) that allows you to automatically modify images so they can be used as backgrounds in the game.
The game requires background images to be tiled in 24x24 pixel chunks, where each chunk is made of 8x8 tiles. Each 8x8 tile has one palette assigned and can use its 16 colors. In total a background can generally have 14 - 16 palettes. Tiles can not use colors from multiple palettes.
SkyTemple can import backgrounds, tilesets and chunksets that follow these restrictions. It reads these from indexed PNGs. Tilequant can convert images into this format, however if the original image contains too many colors per tile, or if a total color limit is exceeded, there will be loss of quality.