Make ISO Image

dd if=/dev/hdx of=filename.iso
The iso file will be created in current directory.

Replace hdx with the device you want to make iso image whether its hard disk, cd/dvd rom or any other disk.
use hdxx for making iso image of a partition.
run fdisk -l if you want to know the partition number.

Make ISO image of a directory
mkisofs -o /tmp/filename.iso /tmp/directory/
The iso file will be created in /tmp directory.

You can also create an ISO image of mulfiple files.
Simply put the files in a directory & run the above command.