If you want an image to completely cover a page in your book project, you're looking for a full bleed layout.
What is a full bleed layout?
A full bleed layout extends to the edges of a page so that the image or background color entirely covers the page with no uncovered edges or white stripes at the edges.
How to create a full bleed layout
It's easy! Make sure your image reaches all the way to the page edge and you've got a full bleed layout. Specific instructions for how to do this in each book-making tool: