codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<style type="text/css">
#thumb {
    margin: 0px;
    padding: 0px;
    height: 100px;
    width: 100px;
    visibility: visible;
}
#image {
    margin: 0;
    padding: 0;
    width: 640px;
    height: 480px;
    visibility: hidden;
}
</style>
</head>

<body>
<div id="thumb">
<a href="#" onclick="getElementById('image').style.visibility='visible'" target="_self">
<img src="02.jpg" width="100" height="100" alt=""/>
</a>
</div>
<div id="image">
<a href="#" onclick="getElementById('image').style.visibility='hidden'" target="_self">
<img src="01.jpg" width="640" height="480" alt=""/>
</a>
</div>
</body>
</html>