You can download select files marked “No Known US Copyright” in the highest resolution the Library has to offer (no request forms or fees). Images can now be used, reused and shared commercially and otherwise. Items may be still subject to rights of privacy, rights of publicity, and other restrictions.
To obtain high resolution .tiff files for all other images (including images/pages from microfilm), you need to request new photography or existing files, for which a fee is payable depending on the context the image will be used in.
The NYPL is also on flickr.com.