Custom Background Image

I have added a custom background color and background image like this:

// Background color and image
$args = array(
	'default-color' => '333333',
	'default-image' => '%s/images/background.jpg',
);
add_theme_support( 'custom-background', $args );

When installing theme, the default background image is displayed. Great.

But there’s no thumbnail of this default background image displayed in the dashboard Theme Customizer (Appearance > Customize > Background Image).

Also when I press Default the default background is gone. So I’m wondering if I added all what’s needed for this function.

Who can help me?

Guido