Couple of alternative ideas for you (or suspect you will find the answer of how to integrate by looking at the code seen in these couple of options)...
a) See a free module that form member PhilJ has kindly provided (option I personally prefer - which is based on
http://mjijackson.com/shadowbox/ ) :- see
http://www.xcartmods.co.uk/x-cart-shadowbox-integration/
b) Search for "lightbox" integration in the forums.... e.g.
http://forum.x-cart.com/showthread.php?t=23858&page=22&highlight=lightbox
Hope these ideas help - cheers, Asiaplay