//just add new image name if new header image is added
headImages=new Array("csheader.png", "drumheader.png", "cableheader.png");

function randomNum ( n ){
  return ( Math.floor ( Math.random ( ) * n) );
}

function changeHead() {
  imgToUse = randomNum(3);  //change parameter value by 1 for each header image
  backgroundString='url(images/' + headImages[imgToUse] +') no-repeat scroll top right';
  document.getElementById('page-header').style.background=backgroundString;
}