On Mac OS X 10.5 it is possible to create ISO files directly from the Disk Utility application but on Tiger, this is not supported. A quick solution is to use the terminal and realising that an ISO file is nothing more than a binary dump of the contents of the disk:
Start by opening a terminal window and insert the CD or DVD you wish to make an image out of. Then type “drutil status” to get information on the CD/DVD reader.
Unmount the disk by typing
$ diskutil unmountDisk /dev/disk1
Disk /dev/disk1 unmounted
Create the ISO file by using the dd command:
$ dd if=/dev/disk1s0 of=image.iso bs=2048
Note: It appears that the device should be stated as above for CDs but similar to /dev/disk1 for DVDs. YMMV