In index.template.php you can replace:
<img id="logo" src="', $context['header_logo_url_html_safe'], '" alt="', $context['forum_name_html_safe'], '" title="', $context['forum_name_html_safe'], '" />
<h1 id="forumtitle">
<a href="', $scripturl, '">', $context['forum_name'], '</a>
</h1>
with:
<h1 id="forumtitle">
<a href="', $scripturl, '">
<img id="logo" src="', $context['header_logo_url_html_safe'], '" alt="', $context['forum_name_html_safe'], '" title="', $context['forum_name_html_safe'], '" />
</a>
</h1>
should do the trick.