StoryPrompt will automatically select a thumbnail for your StoryPrompt however you can edit this and select one from your video or upload your own.
To do this go to your prompt and select Edit.
You will then see two tabs: Prompt and Thumbnail.
Go to Thumbnail and select from the available thumbnails. Alternatively you can upload your own if for example, you'd like a branded thumbnail for your prompt.
You can also text to go over the thumbnail. and select a font and font size. You can also upload an image or add a stock image using Unsplash.