This is the Examine Window. Right click on it to bring up its settings. Drag any side or corner to resize. If the texture is animated, you can create an AVI or quicktime of the animation by clicking on the "create movie" button. Other buttons allow you to change the preview geometry, view the texture tiled, and to view individual channels.
You can drag a DarkTree from the Texture Library to this Examine Window instead of opening another Examine Window.
