I believe the example URL is a little misleading. http://www.urlbackground.jpg seems to be a truncated version of a "proper" example. I think the example should be changed to http://www.urlbackground.com/image.jpg to avoid confusion.
I appreciate that macheide might want to wait until the other Customization FAQs are changed, but I just wanted to point this out :)
Update: Thanks for changing it, it's great now :)