Получаем ссылку на Featured Image в постах

Try this inside the loop:

$feat_image = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
echo '$feat_image';

или вот так:

<?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; ?>