This can also be done at the bottom of the thread page
find:
<if condition="$show['largereplybutton']">
<td class="smallfont"><a href="newreply.php?$session[sessionurl]do=newreply&noquote=1&p=$LASTPOSTID">
replace with:
<td class="smallfont"><a href="newthread.php?$session[sessionurl]do=newthread&f=$forumid" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" /></a>
<if condition="$show['largereplybutton']">
<a href="newreply.php?$session[sessionurl]do=newreply&noquote=1&p=$LASTPOSTID">