To set a desktop wallpaper on a Windows computer, you can follow these steps:

1. Right-click on an empty area of your desktop.
2. Select "Personalize" from the dropdown menu.
3. In the Personalization settings window, click on "Background" in the left sidebar.
4. Choose a picture from the list of available wallpapers or click on "Browse" to select an image from your computer.
5. Once you have selected the desired wallpaper, you can adjust the position, fill, fit, stretch, tile, or center options to customize how the wallpaper is displayed on your desktop.
6. Click on "Save changes" to apply the new wallpaper.

On a Mac computer, you can set a desktop wallpaper by following these steps:

1. Click on the Apple menu in the top-left corner of your screen and select "System Preferences."
2. In the System Preferences window, click on "Desktop & Screen Saver."
3. Select the "Desktop" tab.
4. Choose a folder or collection from the list on the left, then select the image you want to use as your wallpaper on the right.
5. You can adjust the wallpaper position, fill, fit, stretch, tile, or center options to customize how the wallpaper is displayed on your desktop.
6. Close the System Preferences window to apply the new wallpaper.

These steps should help you set a desktop wallpaper on your Windows or Mac computer.