The so-called Bimaran Casket is reputed to be the earliest known representation of the Buddha. It was found inside a stupa in eastern Afghanistan, in the 1830s. The piece itself dates to around 30 BC. You can read the rather fascinating story here, or visit the British Museum, where it now resides. The great pity, of course, is that priceless Buddhist relics were destroyed when the museums in Afghanistan were looted.