To create a world thumbnail, start in preview mode within your world. You can use the three bar icon on your wrist, or the recessed button on your left controller to access the menu. Select the Camera icon and line up the shot you want and press the index finger to capture it. Then wait for the image to save. To apply it as your thumbnail go back into build mode, from the build menu go to the world tab. Under publish, select the update button by the picture, choose your picture and press the back arrow in the top left to save.
Important
Try to make a unique thumbnail that represents your world.
You can change your thumbnail at any time using this method.