There sure is. The key is to first get selected on that video within the middle Inline Editor. To do this, I usually try clicking on the very edge of the editing area to avoid the Text box and to select the video. Once you have the video selected, look on the Ribbon under the Home Tab for Play and Mark. Click this button to open that window. Here, you will have a scrub bar and a few buttons. One button says Mark Thumb. Use the scrub bar to choose what you want to be the Thumbnail image and then click Make Thumb.
Let me know if you have any trouble finding this.