Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 18403)
+++ wp-includes/theme.php	(working copy)
@@ -1654,7 +1654,13 @@
 function get_background_image() {
 	$default = defined('BACKGROUND_IMAGE') ? BACKGROUND_IMAGE : '';
 
-	return get_theme_mod('background_image', $default);
+	$url = get_theme_mod('background_image', $default);
+	if ( is_ssl() ) {
+	        $url = str_replace( 'http://', 'https://', $url );
+	} else {
+	        $url = str_replace( 'https://', 'http://', $url );
+	}
+	return $url;
 }
 
 /**