Find the place you want to add the code in your SHOWTHREAD template and add:
HTML Code:
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" /></a>
Depending on where you add it, you may need div tags or tr/td tags for proper html.