For Mac:

1. Click on the "large" or "small" button.
   
2. Your wallpaper will appear in another window.
   
3. Hold down the option key and click on the wall paper.
   
4. You will be prompted to save the file.
   
5. After saving the file, click on the Apple icon in the top left corner of the screen.
   
6. Click on "Control Panels."
   
7. Click on "Appearance."
   
8. Click on "Desktop."
   
9. Drag the wallpaper file onto the little picture of a screen inside the "Appearance" window.
   
10. Click the "Set Desktop" button in the lower right corner of the "Appearance" window.