I'm having the same problem. I have about 1500 images stored as JPEGs, which have all at some point appeared correctly on iPhoto, but since I downloaded images from a different camera, some of them have been going grey. the images are still stored in the iPhoto library, and when i view them in Preview they display perfectly and I can manipulate and save them, but iPhoto just refuses to have anything to do with them. It will show the thumbnail, but not the full image. If I try to open it up using iPhoto I get a message,
Can anyone help?