facebook “share this” bookmarklet image preview problem

I have a problem with the “share this” button. I use this in my single.php:

<script>function fbs_click() {u=location.href;t=document.title;window.open('http://www.facebook.com/sharer.php?u='+encodeURIComponent(u)+'&t='+encodeURIComponent(t),'sharer','toolbar=0,status=0,width=626,height=436');return false;}</script>

<a href="http://www.facebook.com/share.php?u=<url>" onclick="return fbs_click()" target="_blank">(click to share this post on facebook)</a>

example http://veryserious.org/2009/12/07/crusher

When you use the facebook link though, it can’t find a preview image to use. Sometimes it will work properly and sometimes it won’t Does anyone know the ins and outs of this bookmarklet better than I do and if so, what am I missing?