From the 16 colors of the Commodore Vic-20 to the 512 of the Atari ST or old Apple models: these are just a few of the old-school options you can find on RetroPaint.
To load the image, make sure it's in BMP format, as other formats won't work. Once it's loaded and you've selected the desired color palette, you're free to combine effects, resulting in custom retro images.
You can apply up to three different types at a time, and check the color range of each one separately. With this app, you'll get a historic tour through the evolution of computers over the past few decades.