1. Right-click on the desktop of the macbook and click "Change Desktop Background" in the right-click menu.
2. Then you will enter the background image selection interface, and you will see some system default wallpapers, and you can choose to replace them.
3. If you want to change the background to a personal downloaded picture, you need to click the "+"sign below.
4. Then, in the target folder, select the target picture.
5. Return to the main setting interface, as shown in the figure below.
6. After completing the above settings, you can meet the requirements of the questioner.