attribute to each of your thumbnail images, to change the source of the large image to the full size version associated with that thumbnail.
<img src="simone_thumb.jpg" onClick='document.getElementById("largeImage").src="
<img src="julie_thumb.jpg" onClick='document.getElementById("largeImage").src="j
<img id="largeImage" src="simone.jpg" width="544" height="408"
You can view a working sample of this code here
. For the code sample, I added the small refinement of using the script
tag to print out the directions to click on a thumbnail to see the larger image and using the noscript