To create a world thumbnail, start in preview mode and pull out your selfie stick. Line up the shot you want and snap a picture.

After the image has been saved, head into build mode, and in the World tab, press Update.

Then choose your picture and press the back arrow in the top left to save.

Try making a unique thumbnail that represents your world and also draws interest. You can change your thumbnail at any time using this method, which updates it immediately.