Hello, and welcome in this forum;
simply edit templates/your_template/details.html file.
Find this:
<!-- <br /><br /><a href="{prev_image_url}"><img src="{prev_thumb_file}" border="1"></a> -->
replace by:
<br /><br /><a href="{prev_image_url}"><img src="{prev_thumb_file}" border="1"></a>
And thind that:
<!-- <br /><br /><a href="{next_image_url}"><img src="{next_thumb_file}" border="1"></a> -->
and replace by:
<br /><br /><a href="{next_image_url}"><img src="{next_thumb_file}" border="1"></a>
That's done!