Never done this myself but it seems there are tools that can do this, eg PowerISO from http://www.poweriso.com/
Do they work? I have no idea. I'm sure they will compress ISO image files, but if you are looking for a utility that can compress a bootable ISO file such that it still remains bootable, then the only way to find out is to try.
Other titles available with a Google search for - How to compress an ISO image
You don't say what this 5 GB ISO Image is.
Mark
Hi, I was wondering if you can make a disc image, such as an ISO file, smaller? I mean is it possible to put a 5 gigabyte ISO image onto a 4.7 gigabyte disc?
Any helpful comments will be appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic