We have recently digitized some of our police related special collections, try searching Police and New York in our Digital Collections.
The Library of Congress Prints and Photographs division has digitized images from the NY World Telegraph & Sun Newspaper which includes many photos of police, including mugshots.Their American Memory Project also has image and other resources on police, including a film of a Police Parade from 1899.
The NYC Municipal Archives has digitized some images of Crime and Criminals, which include NYPD mug-shots. There are also over thousands of images found in the search "New York AND Police" in their online gallery.
The New York Public Library Digital Collections has many images related to the NYPD and Police