I think context is very important. This does not exclude the possibility of sin, of course, as with the instance of perverted doctors, priests, and even some parents/siblings, etc. Perversion can occur anywhere, but we can’t avoid the good things - medicine, confession, family, art - simply because of the sinful perversions people do. Pornography is a perversion of the art of modeling, whether photography or otherwise (since photography is not the only kind of pornography; look at the Kama Sutra), but despite this perversion modeling is still an art.
The real question is, when does human art, any art that attempts to replicate the human person (nude or not) become pornographic? And how can we know? I think these are very difficult to answer. Sometimes it is quite obvious, such as the difference between Sistine Chapel nude art or Playboy. But when someone composes a photograph of a naked woman, is it necessarily pornographic simply because it shows breasts or genitalia? The body is not necessarily always sexual when it is exposed, as nudity in the doctor’s office or while taking a bath indicates. And, even if a work of art elicits sexual arousal, does this mean it is pornographic? By definition, pornography objectifies the person or persons involved by portraying sex outside of its marital context, and if it intends to elicit lust. But, how can these intentions be discerned? I think it is quite difficult, especially in certain circumstances, and using quick or pat answers is a bit imprecise, I think.