Poseidon and Amphitrite, originally uploaded by Pachelbel Canon.

Perhaps one of the most beautiful mosaics of antiquity. With its tiny tesserae, it looks much like a painting. It is also widely photographed; there was actually a queue to photograph it the day I visited Herculaneum. I hope my photograph does it justice; it is a lovely piece. Scale is suggested by the bricks below and on either side of it.