How to show most commented posts with thumbnail in WordPress

 
	<?php $popular = new WP_Query('orderby=comment_count&posts_per_page=10'); ?>
		<?php while ($popular->have_posts()) : $popular->the_post(); ?>	
		<?php $thumb_img = get_post_meta($post->ID, 'Image', true); // get an image
			if ($thumb_img) { ?>
		<img src="<?php echo get_post_meta($post->ID, "Image", true); ?>" alt="<?php the_title(); ?>" />
		<?php } else { ?>
		<img src="http://alternative-image.png" alt="" />
		<?php } ?>
		<h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
	<?php endwhile; ?>