This looks like a fine, detailed oil painting showing a battle from the Age of the Fighting Sail. But look closer...
Ponies!
John Eastley composed this beautiful painting entitled "Equestria at War." BUCK 2013, a brony convention held this past summer in Manchester, UK, commissioned and displayed the work. It measures 3x4 feet and is mounted in a gilt frame.
My thanks and a double brohoof to BUCK 2013 organizer Jake Biddulph for providing me with high resolution photos for this post. BUCK 2013 has placed the painting up for sale. You can read more information at the link.