When you lock your Windows 8.1 computer/tablet, it shows the your default logon screen picture. You can enable photo slide show on your lock screen. To do this, make sure you have some photos or picture readily available on your Pictures folders (or anywhere on the computer hard disk).
- Open PC Settings (Move your mouse cursor to bottom-right corner, click settings (gear icon) on the charms bar and click Change PC Settings)
- Click on PC and Devices and them select Lock Screen
- Turn ON the slider for Play a slide show on the lock screen
- If you photos are in Pictures folder, you are done. If not, Add folder in the list.
- Click Back button and close PC Settings (Alt-F4 would be quick way).
That’s all. Enjoy your slide show on the lock screen. 
Anand, the Architect 