The books listed in the middle and right columns include color plates of artworks and a good place to start researching.  The call numbers for the books may be used in both the main and folio (large books) stacks.

Portrait artists by period (look in CONSORT for books on these individuals):

17th Century - France
Le Nain brothers (Antoine, Louis, Mathieu)

17th Century - Netherlands
Peter Paul Rubens
Rembrandt Harmenszoon van Rijn
Johannes Vermeer

18th Century - America
John Singleton Copley
Benjamin West

18th Century - Britain
Thomas Gainsborough
Joshua Reynolds
Johan Zoffany

18th Century - France
Francois Boucher
Jacques-Louis David
Jean-Honoré Nicolas Fragonard
Joseph-Marie Vien

19th Century - America
George Bingham
Mary Cassatt
Thomas Eakins
Winslow Homer

19th Century - Britain
James Tissot
James Abbott McNeill Whistler
Julia Margaret Cameron

19th Century - France
Edgar Degas
Jean-Louis André Théodore Géricault
Jean-Auguste-Dominique Ingres
Edouard Manet
Berthe Morisot