function switch_product_img(divName, totalImgs) {
   for (var i = 1; i <= totalImgs; i++) {
      var showDivName = 'img_' + i;
      var showObj = document.getElementById(showDivName);
      if (showDivName == divName) showObj.style.display = 'block';
      else showObj.style.display = 'none';
      }
   }