UNRECOGNIZED FILES (pictures)

I recently had to reinstall some system software because my computer wouldn't reboot. When I opened up iPhoto 5 nothing was there. I went to a back up and imported the latest Librarty. When I went to import the rest of the photos I got this error message... "The following files could not be imported (they may be an unrecognized file type or the files may not contain vallid data)." All my pictures are the same file type and size and come from the same camera. They all open in the Preview application.