Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 19404)
+++ wp-includes/theme.php	(working copy)
@@ -1678,6 +1678,20 @@
 }
 
 /**
+ * Retrieve background image thumbnail.
+ * Uses defined background image path if theme_mod is empty.
+ *
+ * @since 3.3
+ *
+ * @return string
+ */
+function get_background_image_thumb() {
+	$default = defined( 'BACKGROUND_IMAGE' ) ? BACKGROUND_IMAGE : '';
+
+	return get_theme_mod( 'background_image_thumb', $default );
+}
+
+/**
  * Retrieve value for custom background color.
  *
  * @since 3.0.0
Index: wp-admin/custom-background.php
===================================================================
--- wp-admin/custom-background.php	(revision 19404)
+++ wp-admin/custom-background.php	(working copy)
@@ -196,15 +196,15 @@
 
 if ( get_background_image() ) {
 	// background-image URL must be single quote, see below
-	$background_styles .= ' background-image: url(\'' . get_theme_mod('background_image_thumb', '') . '\');'
+	$background_styles .= ' background-image: url(\'' . get_background_image_thumb() . '\');'
 		. ' background-repeat: ' . get_theme_mod('background_repeat', 'repeat') . ';'
 		. ' background-position: top ' . get_theme_mod('background_position_x', 'left');
 }
 ?>
 <div id="custom-background-image" style="<?php echo $background_styles; ?>"><?php // must be double quote, see above ?>
 <?php if ( get_background_image() ) { ?>
-<img class="custom-background-image" src="<?php echo get_theme_mod('background_image_thumb', ''); ?>" style="visibility:hidden;" alt="" /><br />
-<img class="custom-background-image" src="<?php echo get_theme_mod('background_image_thumb', ''); ?>" style="visibility:hidden;" alt="" />
+<img class="custom-background-image" src="<?php echo get_background_image_thumb(); ?>" style="visibility:hidden;" alt="" /><br />
+<img class="custom-background-image" src="<?php echo get_background_image_thumb(); ?>" style="visibility:hidden;" alt="" />
 <?php } ?>
 </div>
 <?php } ?>