For themes creators. On index.tpl, add

{if !empty($cats_navbar)}
  {assign var=navbar value=$cats_navbar}
  {include file='navigation_bar.tpl'|@get_extent:'navbar'}
{/if}

right after {$CATEGORIES}

And modify existing photos navbar by

{if !empty($thumb_navbar)}
  {assign var=navbar value=$thumb_navbar}
  {include file='navigation_bar.tpl'|@get_extent:'navbar'}
{/if}