Difficult Upgrade to 10.5

[chopper]

I'm trying to upgrade my older Intel iMac to 10.5 from 10.4 so I can run Chrome.

The CD/DVD drive is junk on this unit.

I made a disc image of the 10.5 DVD and put it on the desktop, click install and the computer re-starts--but will not begin the install process. It just goes to the 10.4 install.

Hoping to just do an upgrade and not have to steamroll the 10.4 install.

Any tips on getting the install process to work?

TIA