Mac OS X tiger not installing on iMac G5 late 2005
Probably the medium you have inserted is not bootable by your computer. You can use an USB drive for the reinstallation of Mac OS 10.4 . However, if I recall right, iMac G5 is capable of running 10.5 Leopard as well. It doesn't really matter. I am gonna continue this answer with the assumption of you have a disk image of either 10.4 or 10.5, as well as your mac is currently working or you have access to another mac; since you have mentioned you downloaded the disk image in question itself.
Preparation of installation drive:

Find an USB drive, with 8gb or more storage.
Insert it into the USB port of your computer.
Open up Disk Utility.
Select your drive and go to erase section.
Select &quot;OSX Extended (Journaled) as your disk format and hit erase. (This will delete everything on the thumb-drive, thus make sure that you don't have anything valuable within the USB drive.)
When it finishes, right click on mounted drive and select get info.
Go to &quot;Sharing and Permissions&quot; section of poped up window.
Click the lock and enter your password.
Uncheck the setting &quot;Ignore ownership on this volume&quot;.

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Click the lock again in order to save the change you have just made.
Mount the dmg file of the Mac OSX installation (you can copy from the installation disk or download it from internet.)
Open up the Disk Utility again, this time select the partition you have created in step 5.
Go to restore section.
Select mounted DMG file (not the file itself the mounted image) as the source.
Hit restore.

Booting with the thumb-drive:

Boot/Restart your iMac with &quot;CMD+OPTION+O+F&quot; keys pressed.
Type devalias and press enter.
On the bottom of response from the command in step 2 you will see a drive named ud.
In order to boot from your drive, type boot ud:,\\:tbxi
You should be able to boot from the USB drive.

Installation of Mac OSX:

You should be greeted with the welcome screen.
Proceed the installation by clicking continue.
Select your hard drive, and click options.
Select &quot;Erase and install&quot;. Make sure the format is selected as &quot;OSX Extended (Journaled)&quot;.
Click continue, and it should be installed.

Hope if helps, the process of creating USB drive of 10.4 , 10.5 etc. can be done via any other Mac. Steps are the same.
Ps: Sorry for lack of images during the process. I am using Mojave right now so your screen responses might vary. Only the get info screen is pretty much the same as the &quot;kitten&quot; era of Mac OSX.