There's an undocumented feature: you can also use PNGs and GIFs instead of the JPEGs. But:
- there might be more problems due to color spaces etc. than with JPEGs
- once the map is loaded in main memory, it consumes as much memory as the JPEG based map
You can try it out by putting a PNG file and a position file (XML, KMZ, GSI, GMI, GPX) with the same name into map_images. There must not be a JPEG which has also the same name.