“If you are not rich, you’re not getting rich,” the writer Fran Lebowitz once quipped about life in contemporary America. Judging from “Get the Picture,” Bianca Bosker’s mesmerizing new book about New York’s contemporary art scene, Lebowitz might as well have been talking about cultural capital. If you’re not born with it, you probably won’t amass much of it, because the gatekeepers in this book make it clear that they’re not sharing any wealth. “The art world is the way it is because not everyone has access to it. And not everyone understands it. And that’s sort of what creates interest and intrigue,” a gallerist on the Lower East Side tells the author.