Fig. 1. Penfield, Edward. Cornell. 1908. New York Public Library, https://digitalcollections.nypl.org/items/510d47dc-48d0-a3d9-e040-e00a18064a99.
Penfield, E. (1908). Cornell [Poster]. New York Public Library Art and Architecture Collection, Miriam and Ira D. Wallach Division of Art, Prints and Photographs. (Posters. New York, 1890s-1907. / E. Penfield, Leaf 62). Retrieved from New York Public Library Digital Gallery. http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?strucID=341918&imageID=1131286
Cornell's Code of Academic Integrity covers images. It's important always to cite the work of others.
There are many different ways to cite an image. On the left is one image from an image database, cited according to just a few of the standard formats.
It's important to check style guides to be sure you're citing images properly.
Here are some excellent guides that explain the finer subtleties:
If you're unsure about where to find style guides, try these: