To create an ISO image of a CD or DVD in Linux, you can use the dd command. Open up a terminal and type the following command:
dd if=/dev/cdrom of=/path/toyouriso.iso
This will copy the contents of the folder specified by if to the file specified by of. This can also be used to backup your data from a drive to another too, but be careful, as using a blank disk in if will completely wipe the data of the space it is written to, unless you output it to file (like an ISO file).