function refresh_captcha() {
   var request_url="/login/_ajax-content/?op=captcha";
   YAHOO.util.Connect.asyncRequest('GET', request_url, callback_obj);
}

var callback_obj = {

   success: function(o) {

         var id_obj = document.getElementById('captcha_md5sum_id');
         id_obj.setAttribute("value", o.responseText);

         var image_obj = document.getElementById('captcha_md5sum_image');
         image_obj.setAttribute( "src", "/static/captcha_imgs/" + o.responseText + ".png" );
      },
      timeout: 5000
};