wordpress get post thumbnail

                                    <?php
	echo get_the_post_thumbnail( $post_id, 'thumbnail' );
?>

                                    <?php 
/* RETURNS a full image resource as: <img class="data..." src="data...">
   ALL parameters are optional. Not needed if you are on WP LOOP
*/
echo get_the_post_thumbnail( $the_query->ID, array( 500, 400) ); 
/* USE get_the_post_thumbnail_url() to get only the image url */
?>

                                    <?php if (has_post_thumbnail( $post->ID ) ): ?>
  <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
  <div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">

  </div>
<?php endif; ?>