The photos are, by year in the iphoto library, in whatever format they were in. They are not in any kind of "proprietary" format at all.
Simply go to your home folder, click on your photos folder, right click on iPhoto library and view the package contents. There are two folders called "Modified" and "Originals" which contain all the pictures you have in your all pictures.
Simply click on "originals" then on the year, then on the title of the import batch and you can see whatever pictures were imported at the time, in their original formats.
You don't need iPhoto open (or even need to be able to open the program, if it is for some reason broke) to read any picture on your system. A corruption in your iphoto library file (library.iphoto) in no way makes you lose any pictures on your system... just the indexing within iPhoto.