function makeNav() {

  navImages = document.getElementById('nav').getElementsByTagName('img');
  for (var i = 0; i < navImages.length; i++) {
    var image=navImages[i];
    image.onmouseover=switchImage;
    image.onmouseout=switchImage;
  }
}

function switchImage() {
    var oldSrc = this.src;
    var dot = oldSrc.lastIndexOf('.');
    var numIndex = dot - 1;
    var oldNum = oldSrc.substring(numIndex, dot);
    var newNum;
    if (oldNum == 2) { newNum = 1; } else { newNum = 2; }
    newSrc=oldSrc.substring(0, numIndex) + newNum + ".gif";
    this.src = newSrc;
}