window.onload = changebackground;

function changebackground() {

      var bgs = new Array();
      bgs[0] = "/images/UE_1.jpg";
      bgs[1] = "/images/UE_2.jpg";
      bgs[2] = "/images/UE_3.jpg";
	  bgs[3] = "/images/UE_4.jpg";
	  bgs[4] = "/images/UE_5.jpg";

      var rand = Math.random() * bgs.length;
      var index = Math.floor(rand);

      document.body.style.backgroundColor = '#000000';
      document.body.style.backgroundImage = 'url(' + bgs[index] + ')';
      document.body.style.backgroundRepeat = 'no-repeat';
      document.body.style.backgroundPosition = '50% 0px';
  }