For PC users:
Place mouse cursor over the desired wallpaper. Click the right button on the mouse and select the "set as desktop background" option. This should automatically set the image as your desktop wallpaper.
For Mac users:
Place mouse cursor over the desired wallpaper. Hold down the control key and click the mouse. Select the "use image as desktop" option. The wallpaper should automatically be set as your desktop wallpaper.
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