TILE MYSTERY
Problem solved - thanks to everyone who made suggestions.
I followed the link as suggested by Le Pingouin (easy to follow instructions there), chose the File Folder option, unticked as requested . . . and I'm back to the way things were before this irritant intruded. Even photos on CDs are properly thumbnailed. Of course, I don't know what caused the problem since, in 10 years, I've NEVER been into File Folders. But that's not important. I'm now slightly sane again and can get on with life!