How to upgrade OS on Mac Mini with external USB Drive?

How to upgrade OS on Mac Mini with external USB Drive? We have a G4 Mac Mini, circa 2005, running Mac OS X 10.4 Tiger, and we want to upgrade to 10.5 Leopard. We have a Leopard install disk, but the optical drive in this mini is broken. So we transferred the install disk image to a USB HDD, but now we can't figure out how to boot off it.
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Guest [Entry]

On a PPC Mac such as your G4 Mac Mini, you have to use a Firewire drive. USB booting wasn't really supported before the Intel Macs. You can use a Firewire optical external drive, a Firewire harddrive that has the OS Installer restored to it, put the Mac mini into Target Disk mode and install OS X on a Mac with a working optical drive, or you could even use NetInstall on a MacOS X Server machine.