Hi Ahmed,
For this you have to customize the theme main code.
Just go to theme directory > inc > structure > content-extensions.php on line no:294 you will find <article>
tag below to this tag just paste below code.
<?php if ( has_post_thumbnail()) { ?>
<img style="margin-bottom: 20px;" src="<?php echo esc_url(get_the_post_thumbnail_url(get_the_ID(),'full')); ?>" alt="">
<?php } ?>
Note: We recommended you to create a child theme and paste above code rather than adding it into the theme main code.
Thank you!