iMac g4 OS X installation issue
That flashing icon appears because of 2 cause/reasons

If there is some loose connection between physical drive and motherboard
If file system of OS is damaged


And according to your question i think your problem is of second type. I think you can probably try to zappram(reset RAM) when your system boots press Command+Option+P+R keys, you will then hear the Mac reboot sound(chime) again.

Now press option key wait for some time till system reads DVD contents. Now click on reload or refresh icon if you are lucky than you'll be able to see boot from DVD option