A customer of ours had a problem with changing their wallpaper on Windows 7. There was simply no option to do it when you right clicked on the desktop.

The problem was that the computer was running Windows 7 Starter edition.

Windows 7 Starter does not allow personalization of your desktop or theme. This means no changing the background.

Of course this is a computer and there are ways to do it, it is not that easy to do but it is not really that hard either. So if you really want a background head over to http://www.instructables.com/id/Windows-7-Starter-Easy-way-to-change-wallpaper/ and there are some directions for you.