var bg = new Array();

bg[bg.length]='http://www.pixyup.com/uploads/310306034b64e58b0bd17.png';
bg[bg.length]='http://www.pixyup.com/uploads/211123074ba5f38b25d81.png';
bg[bg.length]='http://www.pixyup.com/uploads/311420294b65839d023e4.png';

document.body.style.backgroundImage='url('+bg[Math.floor(Math.random()*bg.length)]+')';