i need help with facebook code(fbml)

i have a fan page in face book and i wanna do and image that change when the user press like
i tried this but there is a problem..

<script type="text/javascript">
var a = 5;

if (a > 1)
   {
   document.write("<img src='http://www.fredmckinnon.com/myblog/wp-content/uploads/2008/10/true-false.jpg' />")
   }

else
   {
   document.write("<img src='http://www.popwuping.com/stuff/picts/true2.jpg' />")
   }
</script>
<fb:fbml version="1.1">
<fb:visible-to-connection>
var a = 5;
</fb:visible-to-connection>
</fb:fbml>

Code (markup):

im good at programing and ill appreciate if you help me.

thanks, gsw.