AT the Public Library, 1.2 Million Images to Lend Billy Parrott was on the way to a peek at a little treasure when the truth came out. "It looks like he checked out stuff from the library and never returned it," said Mr. Parrott, who is in charge of the New York Public Library's collection of prints and postcards. We can only assume he paid his fine for things not returned. Mr. Parrott was talking about Andy Warhol. If, after all these years, any question remained about Mr. Warhol's settling up, Mr. Parrott considered Mr. Warhol's account balanced by something in the collection. A Warhol. It is a very early Warhol, to be sure. It is a Warhol from before the Campbell's soup can, before the paintings of Marilyn Monroe and Chairman Mao, before the Factory. It is Warhol's Christmas card from 1954. "Warhol was illustrating children's books then," Mr. Parrott said. If dim childhood memories can be relied on anymore, the Warhol postcard looks like something out of the children's magazine Highlights. It had a certain New Yorker magazine sensibility. A comic-book illustration, with "POW!" and "BAM!" in big, dark letters above bad guys fighting somebody like Superman, it is not. The collection that Mr. Parrott, 42, presides over is, the library says, the largest circulating picture collection in the world - over 1.2 million prints and postcards in all, enough to make it a favorite resource for artists, art directors and scenic designers. They can borrow what they find. The collection lends images the way the rest of the library lends books. Every month, from 17,000 to 23,000 images are checked out from the collection, on the third floor of the Mid-Manhattan Library, on Fifth Avenue at 40th Street. With luck, a more or less equal number are returned: original prints, photographs, posters, postcards and illustrations from books, magazines and newspapers. In a digital age, when a Google search can deliver dozens of images in seconds, it has become a throwback to magazines. They are classified under 12,000 subject headings that only a librarian like Mr. Parrott could keep track of. The illustrator Art Spiegelman was so fascinated by the way the collection was filed that he titled an homage to it published in The New Yorker in the 1990s, "Words, Worth a Thousand." Beneath that was this line: "See Also: Images, Unique Circulating Archives of." As Mr. Spiegelman noted, the collection is where "Immanuel Kant rubs shoulders with Wassily Kandinsky and Boris Karloff." But someone looking for them would find them not in the K's but in the P's: "Their images are filed under P (ersonalities) Kan-Kat." "Artists are the collection's heaviest users, but in the 1940s Army intelligence found thousands of photos of Japan invaluable," Mr. Spiegelman wrote. The heaviest use nowadays has to do with costume design, Mr. Parrott said. Broadway costume designers assigned, say, to a play about New York in the 1890s want to be sure the uniform on that police officer in Act II is historically accurate. Producers of cable-television programs with historical flashbacks have the same goal. This being a library, there are rules, though they sound generous. Users may borrow up to 60 pictures at a time. The only requirement is having a library card. For those who lack that, or the patience to go downstairs and get one, there are color and black-and-white copying machines. The collection was started in 1915. Material before 1923 has been digitized, along with special items like the Warhol holiday card and a letter from Diego Rivera. Mr. Parrott, who was appointed manager of the collection late last year, said it had long been his dream job. He is an artist whose work has been displayed at the library, among other places. He attended Pratt Institute and worked at a library as a student. "I realized I was doing what a librarian was doing," he said, so after getting a master's degree in fine arts from Pratt, he got a master's degree in library science. He was assigned to the branch library in Chinatown in 2004 and was promoted to senior librarian at the Jefferson Market branch, a position he also held at the Seward Park branch. In 2010, when the Battery Park City branch opened, he was appointed its chief librarian. Warhol is not the only well-known artist who has used the collection. Rivera, the Mexican artist, sent a letter asking the librarians to accommodate his assistant, who was using the collection to do research for a project Rivera was working on. And yes, Mr. Parrott can tell when the prints have been used in ways that the books in the library's collection are not. "Sometimes," he said, "they come back with paint or little pencil marks."