One more screenshot: editing a layer [fullsize]. An overview image is rendered to give users feedback on layer attributes set through the edit form. The extents of the overview image default to the bounding box of the datastore, and can be customized by setting a bbox property on the layer or its container.
In this case, we're making a Layer of a WMS request to the NASA OnEarth service. This request is for the global_mosaic layer with its default style. This could be changed through the form to other layer/style combinations such as "global_mosaic/visual".