Help with iPhoto please

Hello, I am having a problem with iPhoto / my iMac screen saver where some of my photos that are the correct orientation in iPhoto show as the wrong orientation when my screen saver is running. The only solution I have found to date is to drag the photo to my desktop (when copied to the desktop the pictures are the same orientation as the screen saver rather iPhoto), delete the version in iPhoto and reimport it and the turn the photo. It seems to be only a small percentage of photos that are affected but I would be grateful if anyone can suggest a fix that can be applied to the library rather than to individual photos as I only find out they are affected when they show up on my screensaver.

Thanks in advance,

Adam.