The easiest way to do this is by investigating how it is that particular page is using that image. Depending on the browser, you can do this in a variety of ways.
In Internet Explorer, you can right-click the background and select
Copy Background or
Save Background as... (if it is actually a background image, otherwise there should be
Save Image as...).
In Firefox, you can
View Background Image and save if it's a background image, or
View Image and save if it isn't.
There should be options like this for each browser, with verbiage and exact options varying by version. If not, you'll need to view the CSS or HTML and find the actual URL of the image, as descibed here. Again, by browser you will be provided with a variety of options in doing this (IE's Web Developer option, numerous extensions for Firefox, Chrome's Developer Tools, etc.).