Booting from a USB-CD / DVD / Blu-Ray drive

What undoubtedly happened is that you downloaded an image of a Restore Disk for another model Mac. Installation of that software build isn't supported on your G5 iMac. Check ebay for a retail box "universal installer" DVD, as shown by Allan. Otherwise, you could check online for the gray Restore Disk that shipped with your iMac.

Well, at what exact point did it stop with the exclamation point... if it was after booting from USB but before anything else showed then...

If you can't start up from macOS Recovery - Apple Support

Meaning it didn't work, but not why.:(

It's possible that is the instructions to get a PPC Mac to boot OSX from a USB source.:_

Booting your iBook G4 from a USB stick

So I spent the better half of the night trying to figure out how to make my iBook G4 boot from a USB stick. I have the Leopard ISO, but none of my DVD drives will burn Dual Layer, so I needed to use the USB route.

However, none of the nonsense I found on the Internet could give me proper instructions. The system itself doesn't directly support it (holding down option during power-on doesn't show it either).

So I had to dig into my OpenFirmware roots and do it the old fashion way. So here's the quick tip for the rest of you out there (hopefully Google will eventually pull this up in page ranks so it gets hit first and saves people time).

1. Plug in the USB device where you have copied your bootable system to (I do not cover this part since it's well covered already, google is your friend).
2. Power on your iBook and hold down Command+Option+O+F. This will take you into the OpenFirmware. Scary looking if your not a computer type person.
3. Once you see the screen go white with some text on it, you can release the keys in the previous step.
4. Type "boot ud:,\\:tbxi" and if you're lucky, it will start booting from your USB device. If not, continue on.
5. Type "dev usb0" at the little ">" prompt and hit return.
6. Type "ls". If you see something like "/disk@1", continue, else go to the previous step and use "usb1" instead.
7. If you get here and you haven't seen something like "/disk@1", then you're likely screwed, sorry.
8. Type "dev disk@1" and hit return, and then "pwd" and hit return again. You should see something that looks like "/pci@f2000000/usb1b,1/disk@1". You will use this in the next step.
9. Type "boot /pci@f2000000/usb1b,1/disk@1:,\\:tbxi". This is the device part you got in the last step after typing "pwd" with ":,\\:tbxi" added to the end.
10. Moment of truth, hit enter. You should now be booting into your USB drive. IT WILL BE SLOW SO BE PATIENT

This should also work on other PPC Mac's that don't normally boot from USB, such as G3s.

https://ben-collins.blogspot.com/2010/08/booting-your-ibook-g4-from-usb-stick.html

I have a Blu-Ray disc burnt from a 10.5.6 - iso, that I grabbed from archives.org .

Historically, burned disks and non-Apple downloads have been very unhappy on PowerPC Macs.

Have you looked online for real 10.5 install restore disks? Mine looks like this: