Hi Belinda…this kind of falls outside of theme support, but what I can tell you is that you can do what you want by targeting the page ID and doing custom background image CSS for that page. For example, the link you posted has an ID of “post-85” which if you look at the source code of the page, looks like this:
So you would do css for a background image like this:
article#post-85 {your background css here}
