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Open your css of your style and scroll to where you see "Body (Customized in this Style)" it will be the first box that you see for the css definitions. Now in the box that says "Extra CSS Attributes" place the background code you want to use like this:

background-image: url(images/background.jpg);
background-attachment:fixed;
Be sure to change the images/background.jpg to your correct path where your image for your background was uploaded & give the correct file name for the background. You may have to use a full URL it would look something like this:

background-image: url(http://www.yoursite.com/images/background.jpg);
background-attachment:fixed;