How to change a class CSS with a Greasemonkey/Tampermonkey script?

Question

I\'m trying to set the background image of the body, but only where it uses the class banner_url. The HTML is as follows:

Answer 1

What about something like this ?

document.getElementsByClassName("banner_url")[0] .style.backgroundImage="url('http://www.pxleyes.com/images/contests/kiwis/fullsize/sourceimage.jpg')";

But I must admit I'm not sure to understand the question