Use Post Thumbnail as Background Image

if ( have_posts() ) : while ( have_posts() ) : the_post(); 
    if ( has_post_thumbnail() ) {
        $feat_image_url = wp_get_attachment_url( get_post_thumbnail_id() );
        echo '<div style="background-image:url('.$feat_image_url.');"></div>';
    }
    endwhile;
endif;