the codes listed here is part of the following article : https://www.mwordpress.net/how-to-display-your-most-popular-posts-with-thumbnails-in-wordpress/

popular-thumbnail.php

<?php
$postscount = 5;
$param = array (
	'post__in' 	=> get_post_commented($post_id = NULL),
	'orderby' 	=> 'comment_count',
	'posts_per_page' => $postscount
);
$most_popular = new WP_Query($param);
echo '<ul>';
	while ($most_popular->have_posts()) : $most_popular->the_post();
		?>
		<li>
			<h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
			<?php if ( has_post_thumbnail() ) : ?>
			<div class="thumbnail">
				<?php the_post_thumbnail('thumbnail'); ?>
			</div>
			<?php endif; ?>
		</li>
		<?php
	endwhile;
	wp_reset_query();
echo '</ul>';
?>

popular-timthumb.php

<?php
$postscount = 5;
$param = array (
	'post__in' 	=> get_post_commented($post_id = NULL),
	'orderby' 	=> 'comment_count',
	'posts_per_page' => $postscount
);
$most_popular = new WP_Query($param);
echo '<ul>';
	while ($most_popular->have_posts()) : $most_popular->the_post();
		?>
		<li>
			<h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
			<div class="thumbnail">
				<?php show_thumb(80, 50, 'img-responsive', $post->ID, 100); ?>
			</div>
		</li>
		<?php
	endwhile;
	wp_reset_query();
echo '</ul>';
?>

wordpress-popular-posts.php

<?php
/*
 * Get Most Posts Commented ID
 * @Mouad Achemli
*/
function get_post_commented($post_id = NULL) {
	global $wpdb;
	$date_lookup = '';
	$table = $wpdb->prefix . "comments";

	if ( NULL === $post_id )
        $post_id = get_the_ID();

	// Get list of commented posts in the current mounth
	$sql = "SELECT comment_approved, comment_post_ID, comment_author, comment_author_email, comment_date, comment_content, comment_approved, comment_type, comment_parent
	FROM " . $table . " WHERE comment_date > '%$date_lookup%' AND comment_date >= '".date('Y-m-d', strtotime(date('Y-m-01 00:00:00′)))."' ORDER BY comment_date DESC";

	$post_ids = $wpdb->get_results($sql);
	$output = array();

	foreach ($post_ids as $id) {
		array_push($output,$id->comment_post_ID);
	}

	$result = array_unique($output);
	return $result;
}
?>