If you had upgraded yourself to Windows 8.1 recently then you must know that Windows 8.1 allows you to have slideshow on your lockscreen, you can assign photos or a folder having images which will be shown as slideshow when your computer is locked.
To enable this :-
Press Windows Key + C to bring up your charm menu and then click on Setting –> Change PC Settings,
Now navigate to your PC & Devices –> Lock screen option,
Here you can enable Slideshow, select image or folder which have image and some other options,
Once you had enabled it close it and try it by locking your Windows 8.1 by pressing Windows Key + L.