Thumbnail not displaying

I recently downloaded a slug of pictures from two different cameras. From each camera, I have a small percentage of the pictures that do not display the thumbnail, but only an icon. The media view of these pictures is fine. I tried rebuilding, but had no success. The pictures are all JPEGs. I've successfully downloaded from each of these cameras in the past without having this problem. Suggestions. I also tried taking the card out of one of the cameras and downloading and still have the problem.