code: function(id, img, xy, sz) {
          var imgLocation = OpenLayers.Util.getImageLocation(img);
          var btn = OpenLayers.Util.createAlphaImageDiv(
                                      this.id + "_" + id, 
                                      xy, sz, imgLocation, "absolute");
          btn.style.cursor = "pointer";
          //we want to add the outer div
          this.div.appendChild(btn);
          btn.action = id;
          btn.className = "olButton";
      
          //we want to remember/reference the outer div
          this.buttons.push(btn);
          return btn;
      }
docs: Method: _addButton
  
  Parameters:
  id - {String}
  img - {String}
  xy - {<OpenLayers.Pixel>}
  sz - {<OpenLayers.Size>}
  
  Returns:
  {DOMElement} A Div (an alphaImageDiv, to be precise) that contains the
  image of the button, and has all the proper event handlers set.