first gen emac systems came with either a cd-rom or combo drive... are you sure you have the combo drive? if so, how do you know you have the combo drive? just asking. cd-rom/cd-rw drives will spit out dvd media like you described, that's why i'm asking.
other thought is where did you get the DMG, are you sure it's valid. lots of non-bootable crap floating about on the interwebs... if it's valid, how did you burn it DVD?
the emac will only boot from firewire (hard drive/optical), or in 'target' mode which turns the emac into a really big external drive. you would need another mac with dvd, a firewire port, and firewire cable to pull that off. best of luck