When you click a relief layer in the Project Tree, its panel is displayed below the splitter bar in the project panel.
The thumbnail image on this panel enables you to check the contents of the layer even when it is not currently visible in the 3D view. This is useful when a composite relief is made up of a large number of relief layers. If the thumbnail is not displayed in the panel, click the Information control bar to expand it.
The thumbnail image is a scaled-to-fit view of the relief layer as if it were shown in isolation in the 3D view. The thumbnail view does not use the view orientation currently applied to the 3D view.