Booting G5 From External

I'm having a little problem. I have an iMac G5 without a CD-ROM and I need to reset the administrator password and to do that I have to boot from the OS X CD. What I've done to try to work around that is create an image of the OS X Install Disc 1 and put it on my external hard drive. When I try to boot from the external (option-command-shift-delete), nothing happens and when I let go of the keys I end up in Safe Mode. What I think my problem may be is that there is other data on the external hard drive, so when OS X goes to look for the Install Disc, it sees everything else on the external so it makes the ISO file invalid. Do I need to find an external hard drive with no information on it to act as the Install Disc or is there another way I could/should go about this? Thanks to anybody in advance, this problem has been bugging me for weeks now.

Welcome here, tmaclean.

Seems to me it would be much simpler just to get an external CD or DVD-ROM.

You're very right. The only thing is that this computer isn't mine. I'm doing an internship with my town's public school system and this is one that they recently brought in. We don't have an external cd-rom anywhere within the schools and I don't know anybody with one either. If I had access to one, I surely wouldn't be putting myself through all of this hassle haha.