you dont need to put it between <head></head> just place anywhere in the headerinclude, here is what my code looks like..
Code:
<if condition="THIS_SCRIPT == 'showthread'">
<script type="text/javascript" src="clientscript/prototype.js"></script>
<script type="text/javascript" src="clientscript/scriptaculous.js?load=effects"></script>
<script type="text/javascript" src="clientscript/lightbox.js"></script>
<link rel="stylesheet" href="clientscript/vbulletin_css/lightbox.css" type="text/css" media="screen" /></if>
if you want place the css file under the vbulletin_css directory just change the start of every image url to "../../images.."