Depends on which OS (operating system) you have. Windows 7 / 10 / etc. Generally you can right-click your desktop and choose an option that says "Personalize" then Select "Background"
To use this option you need to have a photo saved of the image you wish to have as your background.
Another way is to search Google for an image, right click the image, and select the option that says "Set as Desktop Background" or "Set as Wallpaper"
