Hi....generally image game files like .cdi version or .nrg still have a dummy file inside...however after have 'changed' the image format into the .nrg one you can use Isobuster to see the content of it or to extract it.
If the image in your case has not the dummy file you can extract it...then add a dummy file and burn it...but it is too long to explain here. So tell me if your image contents a dummy file or not.
All utilities you need are on http://go.to/selfboot.