Is it permissible to sell medical books that contain pictures of women's nakedness, and is the money earned from that considered unlawful, or is selling them considered an ongoing sin?

There is no harm in exchanging or selling medical books that contain images to libraries or medical students, because the images therein are secondary and not the intended purpose of the sale. That which is permissible for a subordinate is not permissible for the principal. What is forbidden is selling them to non-students who intend to acquire the images themselves. Sheikh Ibn 'Uthaymeen permitted the sale of scientific books that include images if the intention is for the benefit (of the text) and not the images themselves, because it is difficult to avoid them, and the use of images in teaching medicine is permissible due to necessity, provided that it is limited to the extent of that necessity.