try hard coding the call to the javascript and css with full urls
eg
<link rel="stylesheet" href="https://www.domain.com/wp-content/plugins/lightbox/css/lightbox.css" type="text/css" media="screen" />
<script type="text/javascript" src="https://www.domain.com/wp-content/plugins/lightbox/js/prototype.js"></script>
<script type="text/javascript" src="https://www.domain.com/wp-content/plugins/lightbox/js/scriptaculous.js?load=effects"></script>
<script type="text/javascript" src="https://www.domain.com/wp-content/plugins/lightbox/js/lightbox.js"></script>
then add rel="lightbox"
to your image code
eg
<a href="https://www.domain.com/wp-content/images/pic.jpg" rel="lightbox" title="pic"> <img src="https://www.domain.com/wp-content/images/pic.jpg" alt="pic"/></a>