The hard drive in my laptop had to be replaced. I had managed to back up the data to an external hard drive (which unfortunately did not also back up program files...). Now, for most, but not all, of the folders in my Pictures library, there are no thumbnails, just a generic picture. If I try to open a picture file, I get an error message saying "Photo Gallery can't open this photo because you do not have permission to access the file location." I checked the permissions, and it appears that I have full permissions for these files, except for "special permissions."
I can create new folder on the desktop and then drag one of the inaccessible files into the new folder. I get a message saying I need to provide administrator permission to move this file. At this point if I click on "continue" the correct thumbnail shows up in the new folder.