When you have two photos on the page, by default, the background is a blurred image of the first photo on the page. If you have one photo per page, it occupies the entire page, thus, no background.
You can always change the color of the background: go to the editing page (by clicking the 
button in the upper, left-hand corner), then click 
.
There is a range of colors that you can choose from for the background: from dark purple to bright yellow, one for any mood you want to set for your story!
When you organize your book on the page with the storyboard (in order to find it, click the 
button in the editing menu), the background color in your book is white, even if you have set a different one. Don’t worry, the storyboard just helps you to organize the pictures, it doesn’t give you the real preview of the book. In order to see what the backgrounds looks like, check out the “Book Preview”.