How to Use a Photo as Wallpaper on a Mac
Make sure that your wallpaper photo is in the JPEG format.
Other file formats may not display correctly.
Choose System Preferences from the Apple menu.
The System Preferences dialog box opens.
Click the Desktop & Screen Saver icon.
The Desktop & Screen Saver dialog box opens.
Click the Desktop button.
You’ll find it inside the Desktop & Screen Saver dialog box.
Locate the folder that contains your photo.
Use the folder list on the left side of the dialog box to do so.
In the thumbnail area of the dialog box, click the thumbnail for the photo you want to use.
You can customize the wallpaper using the other options in the dialog box.
Close the dialog box when you finish.
The photo you selected now appears as your desktop wallpaper.