Mugen screenpacks use a combination of .bmp, .png files for images, and .ini files for configuration. A simple example of adding a dynamic day/night cycle might involve creating several background images and then using .ini files to manage their display.
[Files] spr = system.sff snd = system.snd logo.storyboard = intro.storyboard = select = select.def fight = fight.def common.snd = common.snd mugen screenpack 640x480