You can add a Full Page Image to your manuscript by clicking the three dots next to Add Chapter. Select Full Page Image from the menu that appears, and you’ll see the options for the image appear in the Content Editor and the title Full Page Image will appear on your right navigation pane.
With the Full Page Image open in the Content Editor, you can add a new title to your image by replacing the text 'Fullpage Image' right above the image.
Once your image is added, you'll just need to drag this page to the desired location.