Tileoptimize is a tool that is aimed to size-optimize tiled images for machines with multiple palettes.
Although the output is primarily made for GBA and NDS, it could be easily adjusted to output for other machines aswell.
The application will quantisize the colors of the image to NxM sized palette of M colors spreaded over N palettes,
resulting a more colorful image in smaller tile size with repeating tile and palette combinations to save size.

Download page is HERE.