The video below shows the process of edit a page thumbnail (click the button on the bottom right of the video to expand the view). You can also follow the screen shots and steps below this.
2) Click on the 'Pages' button in the left hand side of the Toolkit.
3) Select the page you want to edit .
4) Click on "Page settings" to edit the options for that page.
5) The page will reload displaying various configuration options. One of the options is titled Thumbnail and allows you to update the thumbnail.Click ADD IMAGE. a pop up will appear showing your library of images.
6) Click select on the image you wish to use as your thumbnail.The pop up window will disappear, placing the image name in the thumbnail text box.