click on gallery thumbs changes feature image

hi i am trying to achieve the following task in a theme that uses the feature image as a background image:
a click on a gallery thumbnail image changes the feature image of the post.

the theme uses the standard wordpress gallery without fancybox.
the feature image is set in this way:

<div id="frontimage">
<?php the_post_thumbnail('picture-page'); ?>
<?php endwhile; else: ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>
</div>

best regards
fredellini