Instructions for setting your desktop wallpaper:
For any wallpaper, click on the resolution size that matches your monitor's resolution. If unsure, choose 1600x1200 if you are using a monitor with a standard shape or 1920x1200 if you have a widescreen monitor. Wait for the image to load completely, then:

Windows users: right click on the image and choose "Set as Background" or "Set as Desktop Background...".

Mac users: Hold down the mouse button over the image until the drop-down menu appears. Choose "download image to disk" (save image to your Mac's hard drive). Open the control panel, click on "Appearance" and then select the Desktop tab. If there is a "remove picture" button, click it to remove the current desktop window. Then click "place picture."
A dialog box will appear. Load new wallpaper from the location you selected to save the image on your computer. Press "Open." Choose "position automatically." Click "Set Desktop." Check desktop for new wallpaper. (You may need to restart your computer for wallpaper to be installed on your desktop.)