Hi, 

Copy file:

/wp-content/themes/thegem/content-page.php

To the child theme and excerpt the code (line 60-66):

<?php
if((get_post_type() == 'post' || get_post_type() == 'thegem_news') && $thegem_featured_content = thegem_get_post_featured_content(get_the_ID(), 'thegem-blog-default', true)) {
echo '<div class="blog-post-image centered-box"-->';
echo $thegem_featured_content;
echo '';
}
?>

add this:

<?php
echo '<div class="blog-post-image centered-box"-->';
if ( has_post_thumbnail() ) {
    the_post_thumbnail();
    }
echo '';
?>

Welcome!