Bing has awesome backgrounds. In Windows Phone 8.1, these can be set as a lock screen background.
Here is how :
Access Settings in Windows Phone 8.1.
Choose “lock screen”.
From the “Background” section, tap on “change photo”.
Select “Bing” from the list as source of background picture.
The lock screen background will now display the very cool Bing images. 🙂
All done.