It's pretty well known that Ancient Greeks and Romans depicted sexual scenes in their art a lot. Greek vases are covered in phallic decorations and show scenes from symposia and bacchic orgies of fantastical lovemaking, prostitution and homosexual relationships. Roman wall paintings in Pompeii are very explicit, including the very famous paintings in the brothels that may or may not advertise the services available.

It's very difficult to link these images to pornography as such, because we have no real evidence that these images were used to help people get off. They are very often interpreted as depictions of every day life, mythology or indeed as menus in the sex industry.

It's worth saying, though, that the idea if visual representations getting you frisky was definitely around in the Classical world. Of course, the famous tale of Pygmalion (Ovid, Metamorphoses Book 10) is all about how a guy got so enamoured of a statue that he clothed it, gave gifts to it, kissed it and, eventually, only went and married it. Pygmalion might've been unusual, but he wasn't alone - Lucian and Pausanias relay the tale of a Knight who fell in love with a statue of Aphrodite at Knidos, locked himself into the temple and got sufficiently intimate to stain the statue. Several of the poems collected in the Priapea (Latin poems about the very phallic god Priapus) suggest that lonely ladies might come and sit on the statues' famous appendages in the night time. Narcissus effectively fell in love with an image of himself so hard he spurned all others. Libanius wrote a serious treatise on "What the Painter Should do if He Falls In Love With the Girl He Has Painted". Basically, the idea that you could be aroused by an image was certainly around at the time - but often in a comic story rather than as a common occurrence.

There are other related stories - people having images made of dead or absent lovers, for example, with some connotations of sexual feelings for the image. I'd highly recommend Maurizio Bettini's A portrait of the lover for a wide range of stories of image-love in Ancient Greek and Roman literature and beyond. You can also check out Eva Keuls's Reign of the Phallus for a great visual overview of penis depictions in the Ancient Greek world.

I appreciate that this only answers your questions insofar as to say that the Greeks and Romans might have had some sexy images, but probably not porn as we know it today. But I hope the incompleteness of the answer is compensated by some interesting tales and is supplemented by someone who knows more...