http://tuxarena.blogspot.gr/2009/03/4-ways-to-create-cddvd-iso-images-in.html
You can use K3b, Brasero, AcetoneISO or... simply the command line:
sudo apt-get install genisoimage
Use it like this:
genisoimage -o my_image.iso my_directory
Or:
genisoimage -o my_image.iso file01 file02 file03
The first command will create an ISO image from the contents of folder my_directory (and it will include it as the root directory), while the second one will create an image containing file01, file02, file03 (and no root directory).
Zoe