Storyboard mode will display an image that you've added to a card, rather than the card's synopsis. In Outline View the image, if present, will be shown on the left side of each row. To enable Storyboard Mode, Tap Settings (gear icon on the upper navigation bar) and flip the switch to ON.
Each card can have a single image associated with it. Tap the add photo button on the card editing screen to use your camera to take a new photo, or select an existing photo from your Photos album.