The desktop background is the image or color that is applied to your desktop. You can also customize the image shown when the screen is locked.
To change the desktop background or the lock screen:
Right-click the desktop and choose Change Background.
Choose Background or Lock Screen.
Click Wallpapers, Pictures, or Colors.
Wallpapers are preconfigured images distributed with your system. Pictures are your own images from your Pictures directory (~/Pictures). Colors are predefined colors chosen by GNOME developers.
Choose an option from the list.
When you are satisfied with your choice, click Select.