window.onload = function() {
var bimages = ["header_1.jpg","header_5.jpg","header_7.jpg","header_8.jpg"];
var rand=Math.floor(Math.random()*4);
newImage = "url(http://www.shtc.org.uk/images/headers/"+bimages[rand]+")";
document.getElementById('table').style.backgroundImage = newImage;
}