After finding the picture closest to the size of your screen, return to that picture by clicking the link. Then...
Windows Users:
Click the picture with the right mouse button. In the pop-up menu, select 'Set as Wallpaper' to save the image as your desktop background image.
Macintosh Users:
Click and hold on the picture. In the pop-up menu, select “Save This Image As...” and save the picture to your hard drive (NOT to your desktop). Under the Apple menu, choose Control Panels/Appearance. Click on the Desktop tab. If you have another background in use, click Remove Picture. Click on Place Picture and select the picture you saved on your hard drive. Click Set Desktop.
To see your new background, close or minimize all application windows.