The earliest surviving depiction of a crucifixion is not work of art but a school boy's scrawl. It dates anywhere from the 1st to 3rd centuries.
It was discovered in 1857 on the Palatine hill in Rome in the remains of a boarding school for imperial page boys built by the Emperor Caligula. It reads in crude misspelled Latin, "Alexamenos worships his god," and shows a figure on the left looking at a crucified figure with a donkey's head seen from behind.
It could well be that Alexamenos was a Christian and this other anonymous boy was mocking him for his beliefs. It could be that Alexamenos may not have been Christian at all, and that showing him praying to a crucified donkey headed god was just another way of insulting him. Alexamenos worships something so low as a crucified donkey man, associating him with foreigners, slaves, criminals, and work animals; the timeless stuff of adolescent insults.
This bit of vandalism sometimes gets over-interpreted (some scholars try to identify it with gnostic sects).
I think it is very useful for revealing the powerful sense of shame that was attached to the crucifixion in ancient times. It reveals the attitude the Romans had of Christ (crucified donkey man).