fastest way:
Open a Terminal and type in:
cat /dev/sr0 > $HOME/test.iso
Or with dd
isoinfo -d -i /dev/sr0 |grep -i -E 'block size|volume size'
dd if=/dev/sr0 of=$HOME/test.iso bs=<blocksize> count=<volumesize>
Test your Image against the CD/DVD
cmp $HOME/test.iso /dev/sr0