You don't have to know where you want to have your pictures or texts or other moments straight away.
Maybe in the beginning you like a picture better on one page, but then want to move it to another? No problem! We all change our opinions sometimes ?
So if you want to move a element in your photo book from one page to another, this is how you do it:
1) Go to your photo book preview
2) Click on the element you want to move
3) Click on Exclude
4) Go to the page you want to add the photo
5) Click on the excluded elements and include it from there
This way you can also just exclude a photo you initially didn't want in your photo book. Not that difficult, right?