There are a bunch of pictures I have in iPhoto 6 from a couple years ago that the thumbnails don't show up on.. but when I double-click on the picture, it opens just fine. The thumbnail just shows up as a white box with a dotted-line box inside of it. I had a similar problem before and I just deleted the pictures and re-imported them.. but I tried that with these and it didn't work. It just said that there was something wrong with the pictures and they couldn't be imported.

It's not a major problem.. but it's a little annoying.