If all you want to do is back up certain files and folders, just burn them to CD as ordinary data files. Remember that recoverability is the key to any backup, and the more complicated you make it, the harder it will be to get the files back if you need them (providing that you can get them at all).
A drive image is the best idea, but that is not really practical with CD's and DVD's, and an external drive is best.
What burning software are you using? If you have packet-writing software (CD drag-and-drop), you can use it, but is will make CD's that are unreadable by many machines and are more difficult to recover. An ordinary ISO CD or DVD is the best idea.