codice:
  <html>

    <head>
      <script type="text/javascript" language="JavaScript">
      <!--

        function changeThumbnail( __hForm )
        {
          var _curSelectionId             = __hForm.elements["imgList"].selectedIndex;
          document.images["thumbImg"].src = __hForm.elements["imgList"].options[_curSelectionId ].value;
        }

      //-->
      </script>

    </head>

    <body>
      
      <form onsubmit="changeThumbnail( this ); return false;">
       
        [img]default.gif[/img]

        



        <select name="imgList">
          <option value="http://www.sito.it/img1.gif">Immagine #1</option>
          <option value="http://www.sito.it/img2.gif">Immagine #2</option>
          <option value="http://www.sito.it/img3.gif">Immagine #3</option>
          <option value="http://www.sito.it/img4.gif">Immagine #4</option>
        </select>

        



        <input type="submit" value=" MOSTRA ANTEPRIMA " />

      </form>

    </body>

  </html>