I think I know what your trying to do. Here's one way to do it.
In your html you can set up your thumbnails like this...
<table>
<tr>
<td colspan="8"><img src="bigImage.jpg" name="big"></td>
</tr>
<tr>
<td><img src="thumb1.jpg" onClick="makeBig('thumb1')"></td>
<td><img src="thumb2.jpg" onClick="makeBig('thumb2')"></td>
<td><img src="thumb3.jpg" onClick="makeBig('thumb3')"></td>
<td><img src="thumb4.jpg" onClick="makeBig('thumb4')"></td>
<td><img src="thumb5.jpg" onClick="makeBig('thumb5')"></td>
<td><img src="thumb6.jpg" onClick="makeBig('thumb6')"></td>
<td><img src="thumb7.jpg" onClick="makeBig('thumb7')"></td>
<td><img src="thumb8.jpg" onClick="makeBig('thumb8')"></td>
</tr>
</table>
then in the <head> section of your html have something like this...
<script language="javascript">
<!--
function makeBig(imgName) {
document.big.src = eval(imgName+"big.src");
}
//-->
</script>
You'll need to sets of images for this to work,
1st set would be
thumb1.jpg to thumb8.jpg
the other set would be
thumb1big.jpg to thumb8big.jpg
I haven't tested this, but it should do the trick.