Hi
You could do it with javascript or flash.
I haven't tested this code, so I don't know if it works, but give it a try.
Put this in your <head> element, below anything else in there:
<script>
function loadImg(url) {
var bigImg = document.getElementById("bigImg");
bigImg.src = url;
}
</script>
Put this kind of thing where you want the thumbnails:
<a href="javascript:void(loadImg('big_1.jpg'))">
<img src="tn_1.jpg" alt="Thumbnail image 1" />
</a>
<a href="javascript:void(loadImg('big_2.jpg'))">
<img src="tn_2.jpg" alt="Thumbnail image 1" />
</a>
And this where you want your full image:
<img src="big_1.jpg" alt="Full image" id="bigImg" />
Without some kind of server-side script you must upload seperate image files for the thumbnail and the full image (for example, here I used big_1.jpg and tn_1.jpg).
Hope this helps.