code: function(url) {
          if (this.useIFrame === true) {
              if (url) {
                  var form = this.createRequestForm();
                  this.frame.appendChild(form);
                  form.submit();
                  this.frame.removeChild(form);
              } else if (this.imgDiv.parentNode === this.frame) {
                  // we don't reuse iframes to avoid caching issues
                  this.frame.removeChild(this.imgDiv);
                  this.imgDiv = null;
              }
          } else {
              OpenLayers.Tile.Image.prototype.setImgSrc.apply(this, arguments);
          }
      }
docs: Method: setImgSrc
  Sets the source for the tile image
  
  Parameters:
  url - {String}