To use desktop pictures on your Mac with OS X:

1. Select the image size that best suits your computer's monitor resolution (i.e. 800x600, 1024x768 or 1280x1024). If you don't know the current resolution of your monitor, check the Appearance Control Panel.
2. Control-click on the link to the picture you'd like to download, and select "Save Image to Disk" from the pop-up menu.
3. Place the downloaded picture in your Pictures folder.
4. Open System Preferences and select Desktop.
5. From the "Collection" pop-up menu, select "Pictures Folder," then select the image you downloaded from the row of preview images.