There are several startup "icons" in the system file. Changing the first one usually changes the one that is shown. But depending on the resolution and color-bit depth on boot will depend on which one is loaded.

I have a totally custom System 8 on my old G3-Wallstreet which was used as an experimental webserver years ago. Its StartUp Screen has Pai-Gon sitting on a monitor asking where the files are. Pai-Gon is a Panda Character from Miss Mab (second on here): http://www.mabsland.com/Adoption.html