Please help me figure out how to Install Leopard from the Install DVD.

[NewsAndHistory]

I need to overwrite Mac OS X Tiger by substituting it with OS X Leopard, but I've been stuck trying to make my computer (iMac G5 2GHz) boot from the install disk/DVD.

 

Each time I restart the computer, and hold the C-key down to make my iMac G5 boot directly from the install DVD, the apple logo shows up without the 'circular gears-animation' spinning underneath it, and once that disappears a folder with a flashing question mark shows up. It stays like this for at least 3 seconds, then the screen goes blank - then it turns grey, and then once again the Mac OS X Tiger's desktop appears, which clearly confirms that I failed to install Leopard even though I held down the C-key for so long.

 

I'm pretty sure the issue isn't with the install DVD because when the install process begins, the disk checker finds no errors. Any help would be greatly appreciated as I'm completely stuck.:aua:

 

P.S. I'm showing this picture (below) so you can see how my iMac screen appears like when that small folder with the '?-mark' shows up (NOTE: a tiny image of the "finder face" also appears up each time I attempt to install Leopard by holding "C" down while my computer is restarting: