Smithsonian
Portraits and so much more! Try the collections search to search across all collections. Results include still images, videos, scanned books & albums, and much more. Subcollections include:
- IRIS: Smithsonian Archives, Manuscripts, and Photographic Collections - SIRIS provides access to archives and libraries across the Smithsonian.
- Department of Anthropology Collections Database - This database provides access to catalog records for the archaeology and ethnology collections.
DNB
Oxford Dictionary of National Biography
Over 60,000 biographies, 72 million words, 11,000 portraits of people who shaped the history of the British Isles and beyond.
This is a great resource for digital images! Each biographical entry includes a list of major likenesses, many of which are reproduced in the entries. Click on each image to get an enlargement and repository & credit information. There are more than 11,000 images of different people in the Oxford DNB. In lists of search results, the portrait icon 
indicates that the entry includes a digital likeness.
The Search tab includes two ways of performing image searches.
In the people search, you can check the image box to restrict your search to people whose likenesses are reproduced in the dictionary.
In the images search, you can search by artist's name, dates, holding institution, etc.
Portrait Image Sites
(Smithsonian Institution's) National Portrait Gallery: Collections Search
Search more than 12,000 records from the collections of the National Portrait Gallery’s paintings, sculpture, prints, drawings, and photographs that document the diversity of individuals who have made significant contributions to the history and culture of the United States. Digitized images are currently being incorporated into the database as copyright research is completed and reproduction rights are cleared.
Portraits of Medical Professionals (Physicians, Nurses, Military and Administrative Personnel)
Images from the History of Medicine, National Library of Medicine
"About one-third of the nearly 70,000 images in IHM are portraits and group portraits in multiple formats. They represent physicians, anatomists, botanist, dentists, pharmacists, nurses, chemists, and others who have contributed through the centuries to medicine and the health sciences." (Fact Sheet)
Portraits in Oxford
Oxford University’s collection of portraits of British and foreign monarchs, leaders of nations, scientists, writers, scholars, and artists.
Looking for portraits?
Cornell subscribes to a number of biographical databases, many of which contain excellent portraits.
American National Biography Online does not provide a specific image search, but entries with images are marked in search results with a camera icon. 
Many of the featured portraits come from Library of Congress collections that may or may not be represented in its American Memory collection.
Biography and Genealogy Master Index
The Advanced search has a portrait check box that allows you to "find citations for sources that include a portrait." This will not lead you directly to the image, but will pre-select likely citations.