Custom_Background::wp_set_background_image

	public function wp_set_background_image() {
		if ( ! current_user_can( 'edit_theme_options' ) || ! isset( $_POST['attachment_id'] ) ) {
			exit;
		}

		$attachment_id = absint( $_POST['attachment_id'] );

		$sizes = array_keys(
			/** This filter is documented in wp-admin/includes/media.php */
			apply_filters(
				'image_size_names_choose',
				array(
					'thumbnail' => __( 'Thumbnail' ),
					'medium'    => __( 'Medium' ),
					'large'     => __( 'Large' ),
					'full'      => __( 'Full Size' ),
				)
			)
		);

		$size = 'thumbnail';
		if ( in_array( $_POST['size'], $sizes, true ) ) {
			$size = esc_attr( $_POST['size'] );
		}

		update_post_meta( $attachment_id, '_wp_attachment_is_custom_background', get_option( 'stylesheet' ) );

		$url       = wp_get_attachment_image_src( $attachment_id, $size );
		$thumbnail = wp_get_attachment_image_src( $attachment_id, 'thumbnail' );
		set_theme_mod( 'background_image', esc_url_raw( $url[0] ) );
		set_theme_mod( 'background_image_thumb', esc_url_raw( $thumbnail[0] ) );
		exit;
	}