If you don’t want to choose one of your own photos, you can choose one of the samples.
If you want to change your account photo, which appears on the lock screen, you can do so in Settings by clicking Accounts and then choosing an option under “Create your picture. "
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.
You can also click on one of the demo photos to set it as your PC’s lock screen.