function changeHeader() {
  var number = Math.floor(Math.random()*12);
  if ( number > 11 || number < 1 ) {
    number = 1
  }
  
  var image = "url(http://www.alliancehealthnetworks.com/images/header-" + number.toString() + ".jpg)";
  document.getElementById("header").style.backgroundImage = image;
}