I think the c key only boot the internal drive. Use startup manager.
The startup manager will list all of your bootable partitions then give you a choice of which to boot. Hold down the option key then power on. Continue holding down the option key until you see the startup manager. This brings up the startup manager. Click on your hd or disc. Click on right arrow key.
Not all external DVD readers will be bootable. Take apart an external firewire hd. Hope it used the right internal interface. Attach dvd drive. etc.
You could buy an external DVD drive. It needs to be firewire and bootable by Mac OS. If you have another PPC with firewire, you could try target Disk mode and installed tiger that way.
You need to figure out the level of your firmware before installing 10.2 or greater. ( The PC name for firmware is BIOS. ) Installing 10.2 with a down level firmware will most likely make your iMac unusable and difficult to fix.
What is Open Firmware?
The firmware on a PPC is called Open Firmware. Open Firmware software receives control when you poweron your machine. It does some hardware testing and some hardware configuration then passes control to your version of Mac OS. It reside on a PROM ( program read only memory ) chip on the logic board.
Figuring out what level of Open Firmware you have?
1) Mac OS 9.x or 8.x, you need to use the Apple System Profiler.
Apple -> Apple System Profiler
2) Mac OS X, use the System Profiler.
Apple -> About This Mac
click on the More Info... tab
click on Hardware
read the Boot ROM Version
3) Open Firmware, boot into Open Firmware.
Power on your iMac while holding down command+option+o+f
The first output line contains the firmware level. Mine reads:
Apple PowerMac4,1 4.1.9f1 BootRom built on 09/14/01 at 13.18.04
Copyright 1994-2001 Apple Computer Inc.
On my machine, I have 4.1.9f1.
What firmware do you need?
http://support.apple.com/kb/HT1395
You can download the firmware from this page:
http://support.apple.com/kb/DL1283
filename: N-iMac_FWUpdate_4.1.9.smi.bin
avoid expanding the file on the download computer.
For a slot loading iMac, this article indicates that you need to be running 9.1 or later version of Mac OS Classic.
http://support.apple.com/kb/DL1283
"The iMac Firmware Update 4.1.9 will only run on iMac computers with slot-loading CD or DVD drives running Mac OS 9.1 or later from a local drive. If you are using Mac OS X you must boot from a local Mac OS 9.1 or later writeable partition (not a CD, or network disk) prior to following the update instructions."
You can download the Mac OS 9 updates from the Apple site.
http://support.apple.com/kb/HT1387?viewlocale=en_US
For tiger, you need 256meg of memory. A slot loader will take 1gig of memory.
You may need to get more memory.
get the 512meg card(s)
http://eshop.macsales.com/shop/apple/memory/iMac/G3_450MHz_500MHz_600MHz_700MHz
Mac OS X 10.4: System requirements
http://support.apple.com/kb/HT1514
PPC 10.4.11 combo update.
http://support.apple.com/kb/dl170