Header images are large pictures that live at the top of your doc. To set a header image:
Hover over the first line of text (Title) in your page's canvas.
Select "Add cover."
After adding the option for a cover image, you may choose from a few options for your photo:
Choose from Unsplash allows you to search a directory of free stock photos for your doc.
Upload Image allows you to upload your own image.
Reposition allows you to drag and move the image so it is framed within the heading,
Remove will entirely remove the cover photo from your doc and return to an image-less page (without a placeholder).