if (document.images) {
  image1on = new Image(); image1on.src = "images/b_submit.gif";
  image2on = new Image(); image2on.src = "images/subo_submit.gif";

  image1off = new Image(); image1off.src = "images/bo_submit.gif";
  image2off = new Image(); image2off.src = "images/sub_submit.gif";

}

function turnOn(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "on.src");
  }
}

function turnOff(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "off.src");
  }
}