Was just looking in my Gallery and found an image with a broken thumbnail. Broken thumbnails seem to be a semi-regular occurence.

Normally if this happens I can use the Gallery 2.x photo edit functionality and;

1. rotate the photo back and forth and the thumbnail rebuilds.
2. or use the crop thumbnail functionality which likewise rebuilds the thumbnail (I'm not aware of a per album thumbnail rebuild option. Let me know if there is)

Despite doing the above, this time the thumbnail stayed broken.

So...

Downloaded the full sized photo, deleted the original photo from Gallery 2.x and then reuploaded - Thumbnail still broken.
Opened in eog (Eye of Gnome) - No errors so I renamed the full sized photo and reuploaded (thinking maybe a stale db record was pointing to an old corrupt image) - Thumbnail still broken.

The fix:
Opened the photo in GIMP and I got an error message "Premature end of image". Used File ==> Save As in GIMP and then reuploaded this new image and the thumbnail worked.