I've often had customers that wanted to be able to configure a unique page for the background of each page on a site. Whether this is a good idea or not is irrelevant.
Here's how I do it.
Add the following text to the index.php file for the tempalte right before the </head> tag. .
$article = JTable::getInstance("content");
$article->load(JRequest::getInt("id")); // Get Article ID
$article_images = $article->get("images"); // Get image parameters
$pictures = json_decode($article_images); // Split the parameters apart
echo "<style> body {background: url('../" . $pictures->{'image_fulltext'} . $pictures->{'image_fulltext_alt'} . "');}</style>";
You may want to set a default background in the regular css file for the site so that no page will have a blank background.