I have an iMac, osx os. Do I already have iphoto on system or is this something I have to add before I can download pictures on a cd?

Check your applications folder to see if you have iphoto. Either way, all you should have to do is drag and drop the pictures you want onto the cd icon on your desktop then burn and eject the disk.

How to Burn a CD on an iMac | eHow.com