In book editor, click on the image you want to remove. Click on the picture or the "image" button above the book to open up the image sidebar. Click the "remove" button in the sidebar. This removes the image from your book, but the image is still in your account.
If you want to remove multiple pictures at once, click "all pages" then select the images you want to move by clicking the circle in the left corner of the images. Then click "remove" in the bar at the top of the screen.