function headerChange()
{
  var header_srcs = ['/images/header/header_1.jpg',
                     '/images/header/header_2.jpg',
                     '/images/header/header_3.jpg']

  var header = new Obj('header_img');


  document.write('<img src="' + header_srcs[Math.floor(Math.random() * header_srcs.length)] + '" width="742" height="236" alt="Department of English" />');
}

function Obj(name)
{
  return document.getElementById(name);
}