featured-image-background.php

if ( ! function_exists( 'magnumopus_featured_image' ) ) :
/**
 * Displays a featured image at the top of single posts and pages and pages with a portfolio or front page template.
 *
 * @since Magnum_Opus 1.0.0
 */
function magnumopus_featured_image() {
	// Return if it's not home and there is no featured image.
	if ( ! is_home() && ! has_post_thumbnail() ) {
		return;
	}

	// Return if it's home and there is no featured image for the blog page.
	if ( is_home() && ! has_post_thumbnail( get_option( 'page_for_posts' ) ) ) {
		return;
	}

	// Get the ID of the blog page. 
	if ( get_option( 'page_for_posts' ) && is_home() ) {
		$id = get_option( 'page_for_posts' );
	}

	// Get the post ID.
	else {
		$id = get_the_ID();
	}

	// Get the post thumbnail url.
	$thumb_url = wp_get_attachment_image_src( get_post_thumbnail_id( $id ), 'magnumopus-featured-img' );

	// Echo the background image.
	echo 'style="background-image: url(' . esc_url( $thumb_url[0] ) . ');"';
}
endif; // magnumopus_featured_image

// Don't forget to add add_image_size( 'magnumopus-featured-img', 1140, 600, true ); to functions.php.