There's no simple solution to this. I have done it, but, for it to work properly in Internet Explorer 9, it requires a DOCTYPE change in XSitePro and that affects other things about your site. And it won't work well in older versions of IE.
Here's a sample: http://www.ablewebs.com/clients/experiment/
You put the image in Page Layout > Page Margins > Background > Image, as usual. But then you need to add some CSS in Other > Global Scripts > HEAD section:
In Other > Global Scripts > Before anything else in the file, add
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
You won't see the changes within XSitePro, you'll have to Preview or Publish the site. And expect other things to now work differently in XSitePro!
There's also an older method that sort of works in older browsers, but it's not as elegant.