I'm a bit late to the punch here, but I highly recommend using palettes graphics. It's limited to 256 colors I'm pretty sure, but that's realistically more than enough, and it saves a ton of memory. However, at the moment, there's not really a good way to convert images into the format without writing a simple script
I plan on making a fully fledged tool eventually than can convert images into the paletted format, and also help you manage the palette registers.