The background image is set in the CSS page. And this has nothing to do with Jade.
So, on the CSS page:
body { background-image: url(/images/img1.jpg); }
Or using a class that is then assigned to one of the jade pattern elements:
.bkimg { background-image: url(/images/img1.jpg); }
and jade:
body.bkimg p this is a fine body
The same applies to any type of element.
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