Index: wp-includes/class-wp-customize-setting.php
===================================================================
--- wp-includes/class-wp-customize-setting.php	(revision 25605)
+++ wp-includes/class-wp-customize-setting.php	(working copy)
@@ -425,15 +425,11 @@
  * @since 3.4.0
  */
 final class WP_Customize_Background_Image_Setting extends WP_Customize_Setting {
-	public $id = 'background_image_thumb';
 
 	/**
 	 * @since 3.4.0
-	 * @uses remove_theme_mod()
 	 *
 	 * @param $value
 	 */
-	public function update( $value ) {
-		remove_theme_mod( 'background_image_thumb' );
-	}
+	public function update( $value ) {}
 }