Change your login screen photo
When you log in or switch users, you will see a list of users with their login photos. You can change your photo to a stock image or an image of your own. You can even take a new login photo with your webcam.
Click the icon at the far right of the menu bar and select System Settings.
Open User Accounts.
Click the picture next to your name. A drop-down gallery will be shown with some stock login photos. If you like one of them, click it to use it for yourself.
If you'd rather use a picture you already have on your computer, click Browse for more pictures.