How to install the desktop pictures...
Mac OS X:
1 • Open the desired picture by clicking on the link that fits the size of your screen.
2 • Once the picture completely loaded, click on it holding the ctrl key down, then chose Save image as... in the contextual menu.
3 • Open System Preferences (usually found in the Dock, but you can also launch it from the «apple» menu). Chose Desktop & Screen Saver, then click on the Desktop tab.
4 • Drag the downloaded file on the desktop thumbnail as shown below:

* You can also store the file in your Pictures folder, then choose it by selecting Pictures folder as shown above.
Windows:
1 • Open the desired picture by clicking on the link that fits the size of your screen.
2 • Once the picture completely loaded, right-click on it, then chose Save as background or wallpaper... in the contextual menu.
or:
2 • Once the picture completely loaded, right-click on it, then chose Save picture to disk... in the contextual menu.
3 • From the Start menu, chose Settings - Control Panels - Display - Background. *For Windows XP users: from the Start menu, chose Control Panels - Appearance and Themes - Display - Desktop.
4 • Chose picture.