Thumbnails are reduced-size versions of images or videos, used to help you recognize and organize them, and serve the same function for images as a normal text index for words. You can turn a part of your scene into a thumbnail in Virtual Tour Net.
To do this, hover over 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 the thumbnail and hit the save button.