I now get this instead, for most (but not all) photos. This is the icon which represents the WLPG application, I think.

Some observations:
- I can see the thumbnails fine in Explorer, which is where a lot of other people have this problem.
- My C:\ drive may have temporarily filled up at some point recently, but then was cleaned up.
- Double-clicking on the photo will display it fine, so I know it's not file corruption or permissions issues.
Thanks