I know this is an old thread, but this might help somebody else searching. I was having the same problem when I noticed the emulation was set for floppy. Since there was no option for CDrom, I set it as custom. Hope this helps.
By the way, another thing I do is burn to a rewritable so if the image doesn't work, I can start again without wasting discs. When you have what you want, copy to a writable CD.