var rotateInterval = 20000;

function startRotation() {
  window.setInterval( rotateHeader, rotateInterval );
}

function rotateHeader() {
  // Generate random number
  var num = Math.floor( Math.random() * 6 );
  
  // Preload the image
  var img = new Image();
  img.src = 'img/headers/header_image'+ num +'.png';
  
  var header = document.getElementById('header_image');
  header.src = img.src;
}