Burn for what purpose? To share with others?
Export the photos to a folder on the Desktop. Burn that with the Finder. Result: a folder of images that any computer can read.
Using iPhoto's Burn command creates an iPhoto Library on the disk and is suitable only for others also using the same version of iPhoto.
This User Tip
has details of the options in the Export dialogue.
Note that rewriteable disks do not necessarily mean that you can append files to them, and require special burning software to do so.
Export the photos out of iPhoto to a folder on the Desktop. Then insert the CD into the optical drive and drag the folder onto the disk's icon on the Desktop. Drag the disk to the Trash bin to begin the burn process.
During the export process if you select Kind = Original you'll get the original file that was imported into iPhoto. If you select Kind = JPEF you'll either get the same original file or, if you've edited the photo, the edited version.