window.onload = function()
{
  var images = ["./images/bg.jpg", "./images/bg2.jpg", "./images/bg3.jpg", "./images/bg4.jpg", "./images/bg5.jpg", "./images/bg6.jpg"];
  var ix = Math.floor(Math.random() * images.length);

  document.body.style.backgroundImage = "url(" + images[ix] + ")";
}