How to get WordPress post featured image URL
Check the code below and let me know if it works for you.
<?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; ?>