codice:
<SCRIPT>
function changeImage(obj,dest)
{
  selection = obj.options[obj.selectedIndex].value;
  document.getElementById(dest).src = selection;
}
</SCRIPT>

<form>
<SELECT SIZE=1 NAME="imagename" onChange="changeImage(this,'IDimmagine')"><OPTION value="html_point_3kb.gif">Primo logo di HTML.it  <OPTION value="html_point_9kb.gif">Secondo logo di HTML.it
</SELECT>

[img]pippo.gif[/img]
ciao