Very annoying: at least since Windows Vista, sometimes a folder full of photos will show some of them as thumbnails and others as icons.
An odd problem with an odder workaround. I tried a number of solutions found on the internet until I tried the second reply (by alphagetti666) in this thread. This only works if you’re on a LAN, but in my case, that's’ the only place the problem showed up.
Basically, don’t navigate to the folder with the photos in it through the mapped drive letter. Instead, navigate by starting with the “network” link in “computer”.
This doesn’t so much fix the problem as the problem never even occurs.