On your Dashboard, your projects are listed as a series of thumbnails with a title and some action buttons such as Edit or Export.
When you create a new project with Moovly, a thumbnail is automatically set. By default, the thumbnail is the frame that corresponds with the position of the Playhead when the last Save was done.
On the Dashboard, go to details page of your project.
In the Player bar, drag the blue dot to that frame of the video that you want to use as the thumbnail: