Non avevo testato....

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

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function pippo(anImage) {
 if (anImage.src.indexOf('i1.jpg') != -1) {
  anImage.src= 'i2.jpg';
 } else {
  anImage.src= 'i1.jpg'; 
 } //  
}
//-->
</script>

</head>
<body>
[img]i1.jpg[/img]
</body>
</html>