Open showthread template in your control panel.

Replace

<normalfont><b><a href="index.php">$bbtitle</A> &gt; <A HREF="forumdisplay.php?forumid=$forumid">$forumtit le</a> &gt; $threadtitle</b></normalfont>

With something like

<table border="0" align="left" valign="top">
<tr>
<td>
<a href="index.php">
<img src="images/open.gif" border="0">
&nbsp;&nbsp;&nbsp;$bbtitle
</a>
<br>
<img src="images/tline.gif" border="0">
<img src="images/open.gif" border="0">
<A HREF="forumdisplay.php?forumid=$forumid">
&nbsp;&nbsp;&nbsp;$forumtitle
</a>
<br>
<img src="images/tline3.gif" border="0">
<img src="images/open.gif" border="0">
&nbsp;&nbsp;&nbsp;$threadtitle
</td>
</tr>
</table>

Edit forumdisplay template

Replace

<normalfont><b><a href="index.php">$bbtitle</A> &gt; $forumtitle</b><

With something like

<table border="0" align="left" valign="top">
<tr>
<td>
<a href="index.php">
<img src="images/open.gif" border="0">
&nbsp;&nbsp;&nbsp;$bbtitle
</a>
<br>
<img src="images/tline.gif" border="0">
<img src="images/open.gif" border="0">
&nbsp;&nbsp;&nbsp;$forumtitle
</td>
</tr>
</table>

Edit paths to point to your images. I highly recommend you to backup your current code (copy n paste to some file) in case you want it back.