A little over a year ago, the New York City art dealer Robert Rogal received a visit to his private showroom from a young woman, who seemed eager to offload a family heirloom. Introducing herself as Karolina Bankowska, she carried a framed painting signed by Andrew Wyeth, resembling the watercolor landscapes the celebrated artist had completed early in his career. In fact, Rogal now believes the painting was a fake — one of at least 200 carefully designed imitations that federal prosecutors say Bankowska, 26, and her father Erwin Bankowski, 50, tried to pass off to unwitting buyers.