var bild_url="http://www.mpip-mainz.mpg.de/www/images/Institut_bw_kl";

if(window.outerWidth){   /*Netscape */
  if(window.outerWidth<900)    document.images['mpipimg'].src=bild_url;
}else{
   if(document.body.offsetWidth ){   /*IE */
      if(document.body.offsetWidth<900 )      document.images['mpipimg'].src =bild_url;
   }
}