MARLBOROUGH College has put Thomas Gainsborough’s painting, The Byam Family, up for auction to raise money for its pound;8 million development plan for an arts centre and a swimming pool. Christie’s expect the 18th-century picture to fetch between pound;3m to pound;5m in the June auction. Attempts to negotiate a private sale to a British gallery or museum have failed so the painting is likely to go to a foreign buyer. The decision was condemned by Charles Saumarez Smith, director of the National Portrait Gallery and a former Marlborough pupil.