By Jan van Eyck (circa 1390–1441) – Web Gallery of Art: Image Info about artwork, Public Domain.
The New York Times features a new article: A Master Work, the Ghent Altarpiece, Reawakens Stroke by Stroke. on the restoration of the 15 century Van Eyck masterpiece, which began in 2012.
For more information, a good article at Wikipedia describes the multifaceted history of the work. To see more details of the piece, visit Closer to Van Eyck: Rediscovering the Ghent Altarpiece.