you can add an image in the story not directly but in terms of dashboards or sheets.
Through the dashboard,
Add an image object and edit the object. Then drag the image dashboard in the story.
Use shapes marks and then create a field with 1 as its value. Now drop the field in shapes shelf. Add an image to be used in tableau repository and select the image. Now edit the size and shape of the image and add the sheet in the story.
Hope it helps.