By Sarah Mills LONDON (Reuters) – Thomas Gainsborough’s 18th century painting “The Blue Boy” goes on show at London’s National Gallery this week, 100 years after it was bought and shipped from Britain to the United States. The portrait of a young boy dressed in blue is on display as part of an exhibition opening on Tuesday – 100 years to the day after it was last shown publicly in London. “‘The Blue Boy’ is not only one of the most famous pictures by Gainsborough, I think it’s one of the most famous pictures in British art altogether,” Gabriele Finaldi, the National Gallery’s director, told Re…