echo get_the_post_thumbnail_url();
<?php // ALL parameters are optional. Not needed if you are on WP LOOP echo get_the_post_thumbnail_url( $the_query->ID, array( 500, 400) ); ?>
<?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 $image[0]; ?>')"> </div> <?php endif; ?>
get_post_thumbnail_id($post)