I use a command line program called NConvert to make the AGS2 themes for Amiga Game Selector. The images are created as standard PNG files in Paint.net and nconvert rescales, reduces the colour palettes and add dither if I need it. It then saves the images as standard IFF ILBM files. I then load them into DPaint to fix the palette so as not to clash with the preview images.
https://imgur.com/a/v2u3JGf