The Blue Boy by Frank Zak
The routine begins with a discussion of how playing cards were originally hand painted and considered pieces of art. A card is then selected and returned to the deck. You then display four postcards, each depicting Gainsborough's The Blue Boy painting. You explain that one of them is a forgery as the spectators immediately spot the obvious and amusing fake.
The postcards are turned over to show Gainsborough painted a back to Blue Boy but no one ever noticed because it was always kept framed. For the finale a postcard that was placed aside on an impromptu easel is revealed with The Blue Boy now holding the selected card.
Can be repeated up to four times with different cards. Comes complete with the necessary postcards and a photo-illustrated booklet. Very easy to do and fun to perform.