I am aware of some nice scholarly work in this space. You may find Shih et al. approach of particular interest [0]. Their approach is very straight forward and

I am aware of some nice scholarly work in this space. You may find Shih et al. approach of particular interest [0]. Their approach is very straight forward and based on image retrieval. They have also reported an accuracy of 99.54% for Adult image detection in their dataset. [0] Shih, J. L., Lee, C. H., & Yang, C. S. (2007). An adult image identification system employing image retrieval technique. Pattern Recognition Letters, 28(16), 2367-2374. Chicago http://sjl.csie.chu.edu.tw/sjl/albums/userpics/10001/An_adult_image_0identification_system_employing_image_retrieval_technique.pdf http://sjl.csie.chu.edu.tw/sjl/albums/userpics/10001/An_adul...