Pornography is harder to define in photography, I'd say simply because of the lack of motion. Regardless of the aesthetic quality of the image however, if there was a clear indication of penetration or stimulation, it's porn. Showing genitals alone doesn't make the image porn. If penetration or stimulation is only suggested, erotica. If it's simply the naked human form, without any 'props' or phallic symbols which might suggest penetration or stimulation - and this could be anything from chair legs to sex toys, it's up to the viewer to find the sexual content. But bare in mind that even the most innocent picture, of an old sweetheart shall we say, can arouse.