Additional custom header image

See TwentyEleven to get the idea: it uses register_default_headers() to add new header images. In your plugin or theme just add:

add_action( 'after_setup_theme', 'wpse_42529_add_header_images' );

function wpse_42529_add_header_images()
{
    register_default_headers( 
        array(
            'a_short_name' => array(
            'url' => '%s/images/headers/a_short_name.jpg',
            'thumbnail_url' => '%s/images/headers/a_short_name-thumbnail.jpg',
            'description' => __( 'Wheel', 'twentyeleven' )
        ),
        'another_image' => array(
            'url' => '%s/images/headers/another_image.jpg',
            'thumbnail_url' => '%s/images/headers/another_image-thumbnail.jpg',
            'description' => __( 'Shore', 'twentyeleven' )
        )
        )
    );
}

The %s will be replaced by the stylesheet directory URI. You don’t have to use that. You can use plugin_dir_url( __FILE__ ) instead of %s.

You can call register_default_headers() multiple times, it works just like a add_default_headers().

Update

To add a new page for custom images similar to the custom header you should extend the class Custom_Image_Header in a theme options page. But this very class is under reconstruction right now – it is almost impossible to write future proof code based on this. I would wait for WordPress 3.4 and build the code on a more stable base.
Well … I will do that, because I need it probably too.

Another way: copy and modify the current class instead of extending it.