if (document.images) {
  var checkbox1 = new Image();
  checkbox1.src = "images/empty_check_box.gif";
  var checkbox2 = new Image();
  checkbox2.src = "images/ticked_check_box.gif";
  var orig = true ;
} 

function swap_image() {
  if (document.images) {
    if (orig) {
      document["checkbox"].src = checkbox2.src
      orig = false;
    } else {
      document["checkbox"].src = checkbox1.src
      orig = true;
    }
  }
}