Today I did some Googling, and it sounded like people were having similar issues when they had installed Aperture. Now, I installed a demo of it a while back, then removed it. So I looked up how to uninstall cleanly, and I learned of a folder in the ~/Library called "Caches" and lo and behold, in the "Desktop" folder in there was a cache of the folders that it was pulling the images from. So I deleted the corresponding cache file, and voila! The thumbnails load cleanly and with no fuss!