Leopard DVD > external HDD question

I'm trying to make a backup copy of my Leopard install DVD. Leopard came preinstalled on my iMac. I'm trying to make an image of Disk 1 to save to to a 10Gb partition on my external HDD as a backup in case I lose the disk.

I then want to use this image of the DVD to install Leopard to another partition on my external drive (which is GUID).

In Disk Utilities I've made a new image of the DVD (tried both .cdr and .dmg formats). when I run the installer from either of these images, I get the prompt to restart, when I do it restarts but returns to my normal desktop (the installation doesn't continue).

Anyone have a suggestion how to install from an image of an install disk? Am I doing the the correct way (I'm a noob).

This is for tiger, but I believe you can adapt it to leopard.

http://www.macfixit.com/article.php?story=20070705094933652

Thanks rman;

I found the updated Leopard version - there are differences from doing it in Tiger, maybe this will work for me (trying it now):

http://www.macfixit.com/article.php?story=20071101112529955