What version of iPhoto?
What has recently changed?
How are you accessing your photos in Preview?
Remember we know nothing about your computer or workflow except what yhou tell us so providing answers to our questions makes a giant different in our ability to help without just making wild guesses
Im using iphoto 11 9.2.1
I did recently get a new iphone 4s and an icloud account even though I don't remember using it.
Also got a drop box account and is synced up with my phone and business computer.
My photos are coming up now even though the format looks a bit different.
I suspect it may be something to do with the iphone as there is a drop down window on the side of the photos that has iphone info now that i hadn't had before..
Thanks for your help