code: function() {
          if (this.imgDiv)  {
              this.clear();
              this.imgDiv = null;
              this.frame = null;
          }
          // don't handle async requests any more
          this.asyncRequestId = null;
          OpenLayers.Tile.prototype.destroy.apply(this, arguments);
      }
docs: APIMethod: destroy
  nullify references to prevent circular references and memory leaks