Thumbnails are reduced-size versions of pictures or videos, used to help in recognizing and organizing them, serving the same role for images as a normal text index does for words. You can make a part of your scene to be a Thumbnail in VideoTours 360.
To get this done, hover around the images in the scene timeline and You will see a Camera icon; click on the camera icon and select the part of the image You want to use for Thumbnail and hit the save button.