Yes, that’s really easy.
Desktop: To delete a picture, video or voice message, just activate “Edit” and click on the x symbol in the upper right corner of the picture. If you do so, the picture, video or voice message disappears and a placeholder appears instead when activating “add media”.
Mobile: To delete a picture, video or voice message, just activate the pencil symbol and click on the x symbol in the upper right corner of the picture. If you do so, the picture, video or voice message disappears and a placeholder appears instead.
By the way: If you don’t want any videos or voice messages in your book, you can simply deactivate them on the editing page.