In that theme's index.template.php:
... the code below is for a Gallery button, change as needed to fit your situation...
Find:
if (in_array($context['current_action'], array('admin', 'ban', 'boardrecount', 'cleanperms', 'detailedversion', 'dumpdb', 'featuresettings', 'featuresettings2', 'findmember', 'maintain', 'manageattachments', 'manageboards', 'managecalendar', 'managesearch', 'membergroups', 'modlog', 'news', 'optimizetables', 'packageget', 'packages', 'permissions', 'pgdownload', 'postsettings', 'regcenter', 'repairboards', 'reports', 'serversettings', 'serversettings2', 'smileys', 'viewErrorLog', 'viewmembers')))
Replace with:
if (in_array($context['current_action'], array('gallery', 'admin', 'ban', 'boardrecount', 'cleanperms', 'detailedversion', 'dumpdb', 'featuresettings', 'featuresettings2', 'findmember', 'maintain', 'manageattachments', 'manageboards', 'managecalendar', 'managesearch', 'membergroups', 'modlog', 'news', 'optimizetables', 'packageget', 'packages', 'permissions', 'pgdownload', 'postsettings', 'regcenter', 'repairboards', 'reports', 'serversettings', 'serversettings2', 'smileys', 'viewErrorLog', 'viewmembers')))
Then
Find:
// the [member] list button
if ($context['allow_memberlist'])
echo ($current_action == 'mlist' || $context['browser']['is_ie4']) ? '<td class="maintab_active_' . $first . '"> </td>' : '' , '
<td valign="top" class="maintab_' , $current_action == 'mlist' ? 'active_back' : 'back' , '">
<a href="', $scripturl, '?action=mlist">' , $txt[331] , '</a>
</td>' , $current_action == 'mlist' ? '<td class="maintab_active_' . $last . '"> </td>' : '';
Add After it:
// the [gallery] button
if ($context['allow_smfgallery_view'])
echo ($current_action == 'gallery' || $context['browser']['is_ie4']) ? '<td class="maintab_active_' . $first . '"> </td>' : '' , '
<td valign="top" class="maintab_' , $current_action == 'gallery' ? 'active_back' : 'back' , '">
<a href="', $scripturl, '?action=gallery">' , $txt['smfgallery_menu'] , '</a>
</td>' , $current_action == 'gallery' ? '<td class="maintab_active_' . $last . '"> </td>' : '';