Ok so I uninstalled Decor8, restarted, set slideshow on Windows 8.1 PC settings, which worked. Then reinstalled Decor8 and lock screen slideshow still worked. Then reset lock screen image on Decor8 settings, and lock screen image was displayed. Then again set slideshow on Windows 8.1 PC settings - locks screen slideshow did not work - lock screen used image previously set on Decor8 appeared on lock screen. So it looks to me that Deco8 setting in this case supersedes Windows 8.1 PC setting. As long as I do not set a lock screen image on Decor8 after reinstalling Decor8, then lock screen slideshow will work.