Posters number among the most venerable traditions of the theater. The delightful Triton Gallery contains a trove from shows past and present, and in sizes from the small cards that appeared in the front windows of ticket brokers to giants used at bus stops and train stations. I particularly admire the work of James McMullan, who for years has created the posters for Lincoln Center Theater, his most recent being the striking artwork for the smash revival of “The King and I.” Triton specializes in his creations. Among the real finds here are posters signed by cast members. To browse the offerings at Triton is to experience the history of Broadway.