Prova cosi....

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function assegna(form){
 var oggetto = document.getElementById(form);
 oggetto.nascosto.value = "ciao";
 alert(oggetto.nascosto.value);
} // function assegna(form)

//-->
</script>

</head>
<body>
<form name="opzioni1" id="opzioni1" method="post" action="dettagli.asp">
 <input type="hidden" name="nascosto">
 <td width="20%" align="center">[img]images.jpg[/img]
      	
</form>
</body>
</html>