If you are stuck with a nero image of a CD, and wante to convert it to a .iso image so that you can create a bootable CD here is a simple solution.
Download and install a tool called nrg2iso
sudo apt-get install nrg2iso
Open a terminal and type
nrg2iso "Source_filneame.nrg" "Destination_filename.iso"
Simple as that, now you have an .iso image ready, have fun.