I just found out that unless lightbox is setup & works off the root of IIS, it does not work.
I have installed a DNN site in a sub directory and marked it as an application, so the URL would be http://localhost/DNNSite/ but lightbox with the products details page has this inserted;
link href="/css/lightbox.css" rel="stylesheet" type="text/css" />
<script type="text/javascript" src="/js/prototype.js"></script>
<script type="text/javascript" src="/js/scriptaculous.js?load=effects,builder"></script>
<script type="text/javascript" src="/js/lightbox.js"></script>
Hence, it is looking for everything at the IIS root level and not the application level. Also the closing X image is also being reference from the root level at that point and not under the actual site folder.
I've worked around this for development purposes by fudging the root level folders, but clearly, it's not uncommon under developement to run DNN under a sub folder and marked as an application in IIS.
John