On my computer, I select booting from the CD at during the POST at start-up.

As the computer is doing the Power On Self Test, I have the option of
either going into the setup to made the CD first choice (which is what
I did), or pressing a function key to select the boot sequence.

As I have a native distrust of dual boot setups, I have not used GRUB.
Which is not to say anything bad about GRUB. I just found it easier
to get hard drive racks for my computer and use my older drives as
test units for new operating systems or ideas.

It sounds like you either saved the image as a file instead of as an image or
your system is not set to boot from CD's/DVD's. Check your BIOS settings for
this under boot sequence.

You may need to go into the BIOS to set it to boot from the CD or by
hitting an F key going into a menu to choose as the computer boots up.

As it boots up watch the scrolling screen and it will tell you which key
to hit to get one of those options.