Click "all pages" to view all the images in your book. Find where you want to insert an image and hover over the gap where you will see a plus sign appear. Click on the plus sign.
From here, select or upload the image you want to add and click the "add" button to insert the picture into your book.
Please note: you may not be able to add a photo to your book if you try to add more pieces than originally paid for. If you get this message, please reach out to email@example.com for assistance.