I tried to give the users the option to switch this feature on or off. Created a column for them to set a yes/no value and wrapped your code like this:
<%contributed($post_id, $post_root_id, $current_user_id)%>
<img src="<%image_url%>/check_posted.gif" border="0" width="13" height="15">
This seems to casue some threads to be marked and others not. Completely inconsistent. Any idea why that is?