Source

File: wp-includes/customize/class-wp-customize-background-image-setting.php

final class WP_Customize_Background_Image_Setting extends WP_Customize_Setting {
	public $id = 'background_image_thumb';

	/**
	 * @since 3.4.0
	 *
	 * @param $value
	 */
	public function update( $value ) {
		remove_theme_mod( 'background_image_thumb' );
	}
}