Try this:
Code:
<html>
<head>
<title>swap images</title>
<meta name="generator" content="BBEdit">
<script language="JavaScript">
<!--
function swap_main(image,name){
document.images["fullsized"].src = "images/"+image;
}
</script>
</head>
<body bgcolor="white">
<form>
<table border="0" cellpadding="0" cellspacing="0" width="310">
<tbody>
<tr align="center" valign="top">
<td height="180" width="155"><a href="javascript:swap_main('image1.jpg')"><img src="images/image1.jpg" border="1" height="150" width="100" onMousedown="swap_main('image1.jpg');"></a></td>
<br><br><br>
<td rowspan="6"> <img src="images/image3.jpg" name="fullsized" border="1" value=""></td>
</tr>
<tr>
<td height="180" width="155"><a href="javascript:swap_main('image2.jpg')"><img src="images/image2.jpg" border="1" height="150" width="100" onMousedown="swap_main('image2.jpg');"></a></td>
</tr>
<tr>
<td height="180" width="155"><a href="javascript:swap_main('image3.jpg')"><img src="images/image3.jpg" border="1" height="150" width="100" onMousedown="swap_main('image3.jpg');"></a></td>
</tr>
<tr>
<td height="180" width="155"><a href="javascript:swap_main('image4.jpg')"><img src="images/image4.jpg" border="1" height="150" width="100" onMousedown="swap_main('image4.jpg');"></a></td>
</tr>
<tr>
<td height="180" width="155"><a href="javascript:swap_main('image5.jpg')"><img src="images/image5.jpg" border="1" height="150" width="100" onMousedown="swap_main('image5.jpg');"></a></td>
</tr>
<tr>
<td height="180" width="155"><a href="javascript:swap_main('image6.jpg')"><img src="images/image6.jpg" border="1" height="150" width="100" onMousedown="swap_main('image6.jpg');"></a></td>
</tr>
</tbody>
</table>
</form>
</body>
</html>