Click on the image you would like to replace. Click on the picture or the "image" button above the book to open up the image sidebar.
Click the "replace" button in the side bar. This will open up a window with image options. Only images that are not already in the book will appear in this window. If you are looking for images that were left out of a book because you had more than the maximum amount of pieces for a book, click "tags" and select the "extra images" tag to see the pictures that were left out of the book.
Select the image you want to add and click the "replace" button to make the change.