making the main menu a High-res image?

so now i know how to make a still background, but now i want to make it a high-res image. i have created an image and inserted it in to mugen, but the palette cosists of about 5 or 6 colours, half of which don't even appear in the original image. is there a way to get a palette with all colours of the original image, and having the *insert plural form of axis here* in fighter factory so the image displays correctly? the image is 1280X720.