In the default theme, only the site name is linked. To link the logo you will have to edit your index.template.php file and change
echo '
<span id="logobox">
<img id="logo" src="', $context['header_logo_url_html_safe'], '" alt="', $context['forum_name_html_safe'], '" title="', $context['forum_name_html_safe'], '" />', empty($settings['site_slogan']) ? '' : '
<span id="siteslogan">' . $settings['site_slogan'] . '</span>', '
</span>
</h1>';
echo '
<span id="logobox">
<a href="', $scripturl, '">
<img id="logo" src="', $context['header_logo_url_html_safe'], '" alt="', $context['forum_name_html_safe'], '" title="', $context['forum_name_html_safe'], '" />', empty($settings['site_slogan']) ? '' : '
<span id="siteslogan">' . $settings['site_slogan'] . '</span>', '
</a>
</span>
</h1>';
That is untested, so I don't know if it will cause any layout issues