If you only want to remove a photo from your book, click on the page that has the photo and go to the "Edit Page" pencil icon. Then head to "Picture" tab. On many types of pages that do not require photos, a "No Picture" option will appear. Selecting that option will remove the photo from the book. Obviously you can't have a photo page without a photo, so if you want to completely remove the page, you always can by clicking the "trashcan" icon next to the preview of the page.
If you want to remove a photo from your account, hover over the thumbnail in the "Pictures" tab. Two icons will appear- the one with the x will delete the image and the one with the pencil will open up the photo editor. Keep in mind that in order to delete a photo from your account, the image must not appear in any of your books.