add_filter( 'body_class', 'sk_singular_body_class' );
/**
 * Adds a css class to the body element
 *
 * @param  array $classes the current body classes
 * @return array $classes modified classes
 */
function sk_singular_body_class( $classes ) {

	if ( is_singular() && has_post_thumbnail() ) {
		$classes[] = 'has-thumbnail';
	}

	return $classes;
}

add_action( 'wp_head', 'sk_singular_body_bkgrd' );
function sk_singular_body_bkgrd() {

	if ( !( is_singular() && has_post_thumbnail() ) ) {
		return;
	} ?>

	<style type="text/css">
		body {
			background-image: url( <?php echo wp_get_attachment_url( get_post_thumbnail_id() ); ?> );
		}
	</style>

<?php }