In Windows 10: Working through the Control Panel options to Appearance & Personalization, then Personalization, then Settings, under Background I select Slideshow. Under "Choose albums for your slideshow" I select the Pictures folder, changing pictures at one minute intervals, Shuffle is Off, "Choose a fit" is Fit. There are probably 3-4 thousand photo files in my Pictures folder (mostly .jpg, and a few .tiff) organized into dozens of folders, with more added all the time.
However, Desktop Background shows photos from only a few folders, while the remaining folders are ignored. In addition, I would like to have only those photos included that have the highest "Rating", but there is no option for this like there was in Windows 7 (Win7 showed only photos rated at 4 or 5 stars). And D.B. doesn't include all the photos in a given folder, it somehow skips some. All features worked satisfactorily in Windows 7. I upgraded to Windows 10 last summer and everything continued to work fine up until a few weeks ago, when it inexplicably began acting aberrantly. I suspect that my problem is a result of unforeseen consequences of a Microsoft automatic update.
I tried a "fix" that I found on the Microsoft Community site -- a command line that takes me back to the Windows 7 setup for D.B. But that didn't work.