let xml_of_image i =
  Element ("image", [],
           [ Element("url",[],[PCData i.image_url]) ;
             Element("title",[],[PCData i.image_title]) ;
             Element("link",[],[PCData i.image_link])
           ] @
           (List.flatten
              [ opt_element
                  (apply_opt string_of_int i.image_width)
                  "width";
                opt_element
                  (apply_opt string_of_int i.image_height)
                  "height";
                opt_element i.image_desc "description" ;
              ]
           )
          )