In order to unlock all of the the editable pages in the book you are making, click on the 'Advance Edit' button at the bottom left of the book builder (see attached screenshot)
You're able to add photos to two of the pages in the book. One is the newspaper (or the scoop) page, where by clicking on the picture of the frog in a party hat, you're able to replace it with a photo of your choice. This is then reproduced on various pages throughout the book, including the fridge page and the stamp page (have a scroll through and see!)
The other one is the quotes page towards the bottom of the book builder which works on the same idea as the newspaper page. Firstly, click on the pencil symbol underneath the page, then click on each quote from the box and you will be able to either drag and drop a photo from your computer or upload a picture your device. There are 9 different quote boxes which you're able to replace in this way.