Hola, sanjo
A ver así:
Código HTML:
<html>
<head>
<script type="text/javascript" language="Javascript">
function amplia(laImg)
{ document.images["grande"].src = laImg.src
}
</script>
</head>
<body>
<table>
<tr>
<td><p><img src="1.gif" width="50" onClick="amplia(this)"></p>
<p><img src="2.gif" width="50" onClick="amplia(this)"></p>
<p><img src="3.gif" width="50" onClick="amplia(this)"></p>
</td>
<td><img src="blanco.gif" name="grande"></td>
</tr>
</table>
</body>
</html> 