I got Windows 8.1 Pro a few days ago and tried to enable lock screen slideshow. However, trying every combination of options in the lock screen page, I am still unable to see a slideshow.
As you can see, my settings page does not include "Let Windows choose pictures for my slide show". My lock screen also doesn't extend to my right monitor. This is in contrast to the options shown on common tutorials on the net. I can also assure you that there are images in the Desktoppr folder, which is used for a slideshow desktop background.