Well, try like this...
you can just rewrite the function and add the extra javascript written below instead of trying to use a custom urchin.js. This is the preferable way, because it saves bandwidth.
What you have to do is this:
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript"></script>
<script src="http://www.yoursite.com/custom_se.js" type="text/javascript"></script>
<script src="http://www.yoursite.com/track-imagesearch.js" type="text/javascript"></script>
<script type="text/javascript">
_uacct = "UA-XXXXXX-X";
urchinTracker();
</script>
Get the two javascript files zipped from here
Hope to be helpful....
