What happened to my picture thumbnails?

I have an odd problem that I've never encountered before. Suddenly all of the folders in my Pictures library have the same thumbnail image for all of them. How do I get them back to normal?

This happens when the cache file where the preview thumbnails are stored becomes corrupt. You can usually use the Disk Cleanup tool to fix the problem. Click Start and type "Disk cleanup" in the search box, then click on it in the Programs displayed. Now be sure "Thumbnails" is checked in the list of "files to delete" and click the "Clean up system files" button. Log off and then log back on. Windows should create a new cache file and the problem should be fixed.