You can easily replace any photo in your book with a different one – from any source.
When browsing your book, on each photo you will see a pencil, a star, and a trash bin.The pencil is to “customize”, the star is to “use as cover”, the trash bin is to “delete”. Click the pencil on the photo you want to replace. On the left, just under the photo, you will see the “Replace picture” button. Click this button and choose the photo that you want to see in your book instead.
As always, all kinds of sources to pick from are available: Facebook, Instagram, Google photos, Flickr, Dropbox, your computer, and more.