Here's the Theme Showcase page for Hatch http://theme.wordpress.com/themes/hatch/ where you can get additional information about working with the theme on WordPress.com. Theme's here are not entirely the same as their original counterpart due to modification to suit the WordPress.com environment.
The Pop Up as you describe sounds like the Photo Carousel, which is the default behavior for images inserted in a post or page using the Gallery feature. You can read more about it here: http://en.support.wordpress.com/images/gallery/
The default background of the Photo Carousel is black but you can change it to white in your Dashboard>Settings>Media Image Photo Carousel. http://en.support.wordpress.com/settings/media-settings/#image-gallery-carousel