Unknown extension image files with no preview available would really like to view?

I have some image files that I would really like to be able to view. They are from a backup cd made years ago with Roxio burn using 2 CDs but one got lost and apparently it was important for making the content on both discs readable and able to be restored to my computer's hard disc. I don't know what file extension these files were now. I tried just to change it to .jpeg and 'trick' a picture viewer into opening them, but it didn't work. Any ideas on how to open the files?