Vorrei sapere come si fa a nascondere l'immagine in javascript?

codice:
<html>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>Nuova pagina 1</title>
</head>
<script language="javascript">
function Nasconde()
{
	document.Form.pippo.value='hidden';
}
</script>
<body>

<form method="POST" action="--WEBBOT-SELF--" name="Form">
  <!--webbot bot="SaveResults" U-File="fpweb:///_private/form_results.txt"
  S-Format="TEXT/CSV" S-Label-Fields="TRUE" -->
  

<input type="button" value="Pulsante" name="B1" onClick="Nasconde()">
  [img]file:///C:/DOCUME~1/riente/IMPOST~1/Temp/FrontPageTempDir/AN01124_.wmf[/img]</p>
</form>

</body>

</html>
Cosa ho sbagliato?