If you know the $Post->ID of the “most popular post”, why not just use get_the_post_thumbnail()
(Codex ref):
<?php echo get_the_post_thumbnail( $id, $size ); ?>
If you know the $Post->ID of the “most popular post”, why not just use get_the_post_thumbnail()
(Codex ref):
<?php echo get_the_post_thumbnail( $id, $size ); ?>