Find that in navbar template:
Code:
<td width="100%"><span class="navbar"><a href="$vboptions[forumhome].php3$session[sessionurl_q]" accesskey="1">$vboptions[bbtitle]</a></span> $navbits[breadcrumb]</td>
Replace it with:
Code:
<td width="100%"><span class="navbar"><a href="$vboptions[forumhome].php3$session[sessionurl_q]" accesskey="1"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></span></td>