Article: You can also do this by swiping in from the right side of the screen, or by hovering the mouse in the screen's top-right corner. It's not possible to change the image that appears in the background of the Windows 8.1 login screen, but you can change the background of the lock screen, which appears before you enter your login information. This method will help you change the background of the lock screen. It's the button with a gear on it at the bottom of the Charms Bar. It's in the left panel. It's near the top of the Personalize window. This can be any photo on your computer, although a high-resolution photo will look better on the lock screen than will a low-resolution one. It's at the bottom-right corner of the window. Doing so will set the photo as your PC's lock screen image, which is where you enter your password to log in.

What is a summary?
Press Win+C to open the Charms menu. Click Settings. Click the Personalize tab. Click the Lock screen tab. Select an image. Click Choose Image.