SInce you give us no informatin only wild guesses can be made
What version of iPhoto and of the OS? What is your exact work flow? Have you tried different media? Do you have the correct media for your drive?
Assuming the correct media for your drive the proper way to burn a disk is to select the photo(s) in iPhoto and export (file menu ==> export) to a desktop folder selecting the desired options - then in the finder right click on the folder and burn to disk
What are you trying to achieve? Are you trying to burn to disk via the iPhoto Share ➙ Burn menu? That produces a mini library on the disk that can be used only by a Mac running iPhoto.
If you want to burn just some image files to a data CD/DVD disk then follow Larry's recommended workflow.
What disk type and manufacturer are your using? What system and iPhoto version are you running? Have you run the OS X Mountain Lion: Use Apple Hardware Test? When did the burning proiblem begin? If you can't burn a disk from the Finder then it's a hardware or system problem and not an iPhoto one.