If you are using Windows 10 and can not see the thumbnails for your photo files (or otherwise), try rebuilding the thumbnail cache. There's a little-known program called the Windows Disk Clean-up Tool that can do the job. Select "This PC" > [Your Drive] > "Properties" and finally check the "thumbnails" option (while unchecking the other options) to rebuild the icons next to your files' names. It may take a while, but just wait it out and you should be good to go. Not everyone names all their files descriptively, so it's really handy to keep thumbnails visible!