When you select a photo on a page, you can remove the photo by clicking the trash icon on the photo tool bar (see below).
Removing a photo from a page only removes it from that page. If it used in any other page, it will remain there. Also when you delete a photo, its frame or container remains on the page. You can either ignore it or delete the frame (an empty frame won't appear when printed). The image below shows an empty photo space on a book page after the photo has been removed. If you click the trash can again it will delete the frame too.
If you want to completely delete a photo from your project, after removing all instances of it from your book, you need to delete it from the photo well in the project. For more information about the photo well, see Viewing the images you've used in your photo project.
Note: deleting a photo from the project doesn't remove it from your photo library.