http://cdimage.ubuntu.com/lubuntu/releases/raring/release/lubuntu-13.04-desktop-powerpc.iso
Boot ISO (Option-C) when power on or 'c' if yaboot is already installed.
At the boot: prompt type:
live video=radeonfb:1024x768-32@60
https://help.ubuntu.com/community/Lubuntu/Documentation/FAQ/PPC
Install!