The majestic renovation of the New York Public Library’s Mid-Manhattan branch, now known as the Stavros Niarchos Foundation Library—new atrium, new children’s room, new roof deck, new name—also resulted in a homecoming of one of the N.Y.P.L.’s underknown marvels. That marvel, the Picture Collection, is an archive of more than a million loose, printed images, organized in folders alphabetized from Abacus to Zoology, which are available for visitors—immigrants, historians, illustrators, set designers, and beyond—to sift through and check out, like books. For many years, beginning in 1915, the collection was in Room 100 of the Fifth Avenue research library; now, after decades at Mid-Manhattan, it’s there again. On a recent Wednesday, the head of the library’s art division, Joshua Chuang, met up there with the photographer Arnold Hinton and the…