LONDON.- Every month during Dulwich Picture Gallery’s Bicentenary celebration year a spectacular masterpiece will hang on the end wall of the Gallery’s enfilade. February sees the arrival of El bufón Don Sebastián de Morra by Diego Rodriguez de Silva y Velázquez (1599 -1660). Noel Desenfans and Sir Francis Bourgeois, the two founders of Dulwich Picture Gallery, believed that they owned a portrait of King Phillip IV of Spain by Velázquez, painted at the time of the Battle of Fraga in 1644. In fact that canvas is a seventeenth-century copy of the original by Velázquez, now in the Frick Collection in New York. El bufón Don Sebastián was painted by the Spanish artist around the same time as the Fraga portrait of the King and is one of the painter’s most arresting and memorable works. The dwarf Don Sebastián de Morra was the buffoon and servant to Philip IV’s brother, the Cardinal-Infante Don F