Show gallery featured thumbnail

ARCHIVE-GALLERY.php

<?php $cats = get_galleries() ?>
<ul>
    <?php foreach ( $cats as $cat ) : ?>
    <li>
    <a href="<?php the_gallery_link($cat) ?>">
            <?php the_gallery_featured_image($cat) ?>
        </a>

    <a href="<?php the_gallery_link($cat) ?>">
            <?php echo $cat->name ?>
        </a>
    </li>
    <?php endforeach ?>
</ul>