eBook is one of the templates that you can use to make a new book or shell book. The default page size and orientation is Device 16x9 Landscape.
It has these pages: Front Cover, Credits Page, Title Page, and Back Cover pages. In addition, it gives you a number of different templates. You see them when you add a page.
You can add a page (an inside page) to the book.
You can choose a page that puts a picture in the desired location on the page.
You can choose a page that is only for words or only for a picture.
You can customize any of the pages, or you can use the Custom page template to design your own page (example).