var imgpath = "images/shoes/med/";  

function showImage(imgName) {
  document.images["mainImg"].src = imgpath+imgName+".jpg";

  var elmt = document.imgMenu;
  for (var i = 0; i < elmt; i++) {
    if (elmt.name == imgName) {
      elmt.selectedIndex = i;
      break;
    }
  }
}