Venus Anadyomene

I was reading, in the course of my researches, some Victorian pornography. At one point, the mighty Walter pauses in the middle of his exertions and says of his circumstances that 'it was a picture worthy of any Apelles.' The above is the only picture I can find which has anything to do with the painter Apelles, and it is only a presumed copy, from Pompeii, of one of his pictures. Lemprière has the following:

'Apelles, a celebrated painter of Cos, or, as others say, of Ephesus or Colophon, son of Pithius. He lived in the age of Alexander the Great, who honoured him so much that he forbade any man but Apelles to draw his picture. He was so attentive to his profession that he never spent a day without employing his pencil, whence the proverb of Nulla dies sin lineâ. His most perfect picture was Venus Anadyomene, which was not totally finished when the painter died. He made a painting of Alexander holding thunder in his hand, so much like life that Pliny, who saw it, says that the hand of the king with the thunder seemed to come out of the picture. This picture was placed in Diana's temple at Ephesus. He made another of Alexander, but the king expressed not much satisfaction at the sight of it: and at that moment a horse, passing by, neighed at the horse which was represented in the piece, supposing it to be alive: upon which the painter said, "One would imagine that the horse is a better judge of painting than your Majesty." When Alexander ordered him to draw the picture of Campaspe, one of his mistresses, Apelles became enamoured of her, and the king permitted him to marry her. He wrote three volumes upon painting, which were still extant in the age of Pliny. It is said that he was accused in Egypt of conspiring against the life of Ptolemy, and that he would have been put to death had not the real conspirator discovered himself, and saved the painter. Apelles never put his name to any pictures but three: a sleeping Venus, Venus Anadyomene, and an Alexander. The proverb of Ne sutor ultra crepidam is attributed to him by some.'

So the question is: how does a painter, none of whose works survive, and copies of whose extant work don't make him seem particularly filthy, become a byword for pornochromism? The fact that he never spent a day without employing his pencil is probably (har-har) a dead end.