A thumbnail is a small representation of an image that is used to identify a file by its contents intended to make it faster to manage a group of larger images.
To create a thumbnail, create a snapshot of the image you would like to be displayed as the thumbnail and click "save to product list".
The image will display in the Product list section as seen below:
Right click on the desired snapshot and choose Set as Thumbnail
Your updated thumbnail will be displayed in the Project Panel.