dà un errore perchè manca l'elemento con id="citta". Io ho scritto così (non so se è quello che volevi)

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="it" xml:lang="it">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
        <title>xhtml 1.0</title>
        
        <script type="text/javascript">
        // <![CDATA[
        
        function WebCam() {
        var vettore = new Array (
        new Array("Moncalieri (TO)", "http://www.nimbus.it/moncalieri/monc_vista.jpg"),
        new Array("Isola d' Elba", "http://www.elbalink.it/webcam/webcamview.php?n=4"),
        new Array("Vista di Genova", "http://www.stefanome.it/current.jpg"),
        new Array("Grado (UD)", "http://80.105.84.114/webcam2.php"),
        new Array("Bologna", "http://www.bolognameteo.it/broadcast.jpg"),
        new Array("Brescia", "http://www.starrylink.it/webcam/brescia/brescia1M.jpg"),
        new Array("Canazei (TN)", "http://www.dolomitiwebcam.com/vernel/mega.jpg"),
        new Array("Cattolica (RN)", "http://www.riminibeach.it/assets/images/cattolica/cattolica1.jpg"),
        new Array("Cuneo", "http://www.provincia.cuneo.it/_images/webcam/netcam2.jpg.tmp"),
        new Array("Etna", "http://www.etnaweb.net/EtnaCam/WebCam2/ultima.jpg"),
        new Array("Folgaria (TN)", "http://82.105.110.187/axis-cgi/jpg/image.cgi?compression=25"),
        new Array("Napoli", "http://www.salernometeo.it/Webcam/napoli/currentsmall.jpg"),
        new Array("Pontebba (UD)", "http://www.comunitamontanadelgemonese.it/UD11/pontebba.jpg"));

        var rnd = Math.floor(Math.random() * vettore.length); //Numero random intero compreso tra 0 e il limite superiore dell'array
    
        if (document.getElementById('citta'))
        document.getElementById('citta').innerHTML = vettore[rnd][0];
        document.getElementById('immagine').src = vettore[rnd][1];
        }
        </script>



        <script type="text/javascript">
        <!--

        //set image paths
        src = ["http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16066.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16105.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16140.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16230.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16240.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16289.gif", "http://banners.wunderground.com/weathersticker/big2_metric_cond/language/www/global/stations/16405.gif"]

        //set corresponding urls
        url = ["http://www.wunderground.com/global/stations/16066.html", "http://www.wunderground.com/global/stations/16105.html", "http://www.wunderground.com/global/stations/16140.html", "http://www.wunderground.com/global/stations/16230.html", "http://www.wunderground.com/global/stations/16240.html", "http://www.wunderground.com/global/stations/16289.html", "http://www.wunderground.com/global/stations/16405.html"]

        //set duration for each image
        duration = 2;

        //Please do not edit below
        ads=[]; ct=0;
        function switchAd() {
        var n=(ct+1)%src.length;
        if (ads[n] && (ads[n].complete || ads[n].complete==null)) {
        document["Ad_Image"].src = ads[ct=n].src;
        }
        ads[n=(ct+1)%src.length] = new Image;
        ads[n].src = src[n];
        setTimeout("switchAd()",duration*1000);
        }
        function doLink(){
        location.href = url[ct];
        } onload = function(){
        if (document.images)
        switchAd();
        }
        //-->
        </script>

<body onload="WebCam();">

<img id="immagine" />

        


</body>

</html>

Ciao