The image is not graven, by the way. Most are molded. And “graven image” is the same as “idol”. Pictures or statutes of Mary are not idols, and we “idolize” Mary not as a goddess, but more like moderns do superstars, an extremely gifted human being. Except our “idol” does not have clay feet.
This, I think, is the point. The prayers are directed to Mary, not to the pictures of Mary. The pictures are easily replaced and are not the object of worship.