Pornography is culturally defined, and since we have little understanding of what Paleolithic rock art meant, there is no way of knowing whether it was regarded as pornographic. That said, I think it's a pretty good guess that it wasn't. This scene of the dead man in Lascaux, France, shows an erection, but I doubt it is for an obscene purpose. Is this simply to indicate that the subject is male, or has his neck broken, causing an involuntary erection? Again, we cannot know.

In addition, there are examples of Paleolithic art - particularly mobility art - of the so-called "Venus figurines". Again, we don't know the purpose or significance, but those who have examined them have tended to look to ritual or belief-system explanations rather than anything to do with pornography.

Thanks for the great answers! I suppose it is hard to define what I mean. I just wonder what the earliest examples of obscene graffiti was. Do we know when the first "porn" was invented? Would a caveman feel inclined to paint a woman with huge jugs on his cave wall to stare at in the lonely winter months?

As a more general followup: Instead of porn specifically, were there paintings of sex or sexuality in general? Do we have paintings of what appear to be two humans copulating, or any other clearly sexual act?

I know this was mentioned in comments below, but I would love to see it expanded upon.