Overlays
Overlays were originally created so we could stamp them onto movie thumbs.
There is a default overlay image that can be stamped onto all galleries, this setting is on the settings page. The image must be a transparent png file, which means you need imagemagick with PNG capability installed.
Each category can have it's own overlay image assigned to it.
Overlays can be disabled through a check box in gallery review, on a per gallery basis. They should stamp onto galleries automatically.
If you have more questions about image overlays, then you can