interface customization was one of those things that was higher priority then adding additional sources to the video module
at the moment the only way to customize the NBB images is to manually edit/replace them by digging through the app bundle (as well as the module bundles).
be warned that the color change slider in the prefs will have an affect on any of the red images you edit/replace so if you want custom images to use with a different color then the default red you will have to compensate with a hue adjustment.
one of the first things on the list after 1.0 is out the door is to expand the theming capabilities of NBB. Image substitution will be one of the first things in, and your theme/skin will be able to blacklist images from the coloring adjustment so that won't be an issue in the future.
I don't know if you already saw or not but hanzov69 has a made a custom clock graphic for the MINI freely available
here