Okay, I didn't think that would work because the link you obtain doesn't open a webpage showing the image like normal image urls do. Anyhow, I got the link and tried to follow the instructions on the article to add a background image. In my guinea pig story entitled "Story10", I added the following line on the first and only page:
<body background="http://chooseyourstory.com/resources/images/user/56027?56027">
where the URL is the one for the image I'm using. According to the article, this should set the background image of the page, but no image shows up in the preview.