If you want to set that as the background for your website, it's as simple as writing it in the CSS file as:
html {
background-image: url(http://oi61.tinypic.com/14o3c3k.jpg)
}
or setting it via jQuery (you tagged your question with jQuery, so I assume you want to know how to do it that way)
$("body").parent().css("background-image", "url(" + http://oi61.tinypic.com/14o3c3k.jpg + ")")
//you can also use $("html")… I've just seen cases where that failed for people