If you already have a fancy background and you want to change it, or you are being forced to change it because the background you already added has mysteriously disappeared, then this is the post for you.
The first thing you need to do is remove the HTML code for the old background.
To do this, click on Design and then click on Page Elements and locate the Gadget that has the HTML code for the old background. Click on Edit and then, when the pop-up opens, click on Remove.
Now upload your new background as shown in this post.