The file `large-penguin.mbicon' (mbicon == MultiBoot Icon) is an icon that will replace the disk-icon-with-small-penguin-badge in the OpenFirmware multi-boot screen (hold down option on AGP G4s and iBooks). In order to use this copy an uncompressed copy somewhere, and add `icon=/path/to/large-penguin.mbicon' or whatever to /etc/yaboot.conf and run ybin. You can add other types of icons so long as you put the proper bootinfo tags around them. This is unsupported and you are on your own.