Michael Bedesem's great program, MidiPlayer, can change and/or assign any of the Yamaha icons to any style or midi file. While it's main purpose is to play styles and midis on your computer, it does already have this icon editing function. Not to discourage you, but you might take a look at what he has done:
http://www.svpworld.com/psr_software.htm#stylemanager

Tom G.