The Nation's Capitol
National Gallery of Art -  searchable database - 14 Sargents
National Portrait Gallery - Washington, DC
National Portrait Gallery Hall of Presidents
National Museum of Women in the Arts - Washington, DC
Corcoran Gallery of Art - Washington, DC
Kreeger Museum - Washington, DC
Library of Congress Exhibitions - Washington, DC
Freer Gallery of Art
Smithsonian American Art Museum

The Northeast
Philadelphia Museum of Art - Philadelphia, Pennsylvania
Pennsylvania Academy of Fine Arts - Philadelphia, Pennsylvania
Brandywine River Museum - Chadds Ford, Pennsylvania
Palmer Museum of Art - University Park, Pennsylvania
Metropolitan Museum of Art - New York City
Brooklyn Museum of Art - Brooklyn, New York
The Jewish Museum - New York, New York
The Morgan Library - New York, New York
The Hispanic Society - New York, New York
Albright-Knox Art Gallery - Buffalo, New York
Neuberger Museum of Art - Purchase, New York
The Frick Collection - New York, New York
Museum of Fine Arts, Boston - Boston, Massachusetts
Peabody Essex Museum - Salem, Massachusetts
Yale Center for British Art - New Haven, Connecticut
New Britain Museum of American Art - New Britain, Connecticut
Wadsworth Atheneum Museum of Art - Hartford, Connecticut
Harvard University Art Museums - Cambridge, Massachusetts
Isabella Stewart Gardner Museum - Boston, Massachusetts
The Newark Museum - Newark, New Jersey
Montclair Art Museum - Montclair, New Jersey
Hood Museum of Art - Hancock, New Hampshire
Worcester Art Museum - Worcester, Massachusetts

The Southeast
Baltimore Museum of Art - Baltimore, Maryland
The Walters Art Gallery - Baltimore, Maryland
The Atlanta Museum of High Art - Atlanta, Georgia
Telfair Museum of Art - Savannah, Georgia
Georgia Museum of Art - Athens, Georgia
Morris Museum of Art - Augusta, Georgia
Hunter Museum of American Art - Chattanooga, Tennessee
        -dedicated exclusively to Southern artists
Virginia Museum of Fine Arts - Richmond, Virginia
North Carolina Museum of Art - Raleigh, North Carolina
The Reynolda House of American Art - Winston-Salem, North Carolina
Columbia Museum of Art - Columbia, South Carolina
The Chrysler Museum of Art and Historic Houses - Norfolk, Virginia
Museum of Fine Arts, Houston - Houston, Texas
Dallas Museum of Art - Dallas, Texas
San Antonio Museum of Art - San Antonio, Texas
The Mississippi Museum of Art - Jackson, Mississippi
The Montgomery Museum of Fine Art - Montgomery, Alabama
Birmingham Museum of Art - Birmingham, Alabama
St. Petersburg Museum of Fine Arts - St. Petersburg, Florida
Ringling Museum of Art - Sarasota, Florida
Cornell Fine Arts Museum - Winter Park, Florida
Norton Museum of Art - West Palm Beach, Florida
New Orleans Museum of Art - New Orleans
Lowe Art Museum - Miami, Florida

The Midwest
The Amarillo Museum of Art - Amarillo, Texas
Amon Carter Museum - Fort Worth, Texas
The Art Institute of Chicago - Chicago, Illinois
St. Louis Art Museum - St. Louis, Missouri
Woodcock Museum - St. Louis, Missouri
Nelson-Atkins Museum of Art - Kansas City, Missouri
The Detroit Institute of Arts Museum - Detroit, Michigan
Milwaukee Art Museum - Milwaukee, Wisconsin
The Minnesota Institute of Arts - Minneapolis, Minnesota
Minnesota Museum of American Art - St. Paul, Minnesota
Indianapolis Museum of Art - Indianapolis, Indiana
Indiana University Art Museum - Bloomington, Indiana
The Cleveland Museum of Art - Cleveland, Ohio
Cincinnati Art Museum - Cincinnati, Ohio
The Butler Institute of American Art - Youngstown, Ohio
Columbus Museum of Art - Columbus, Ohio
Akron Art Museum - Akron, Ohio
Davenport Museum of Art - Davenport, Iowa

The West
The J. Paul Getty Museum - Los Angeles, California
Fine Arts Museum of San Francisco - San Francisco, California
       -searchable database of 75,000 images; create your own online gallery
Crocker Art Museum - Sacramento, California
Santa Barbara Museum of Art- Santa Barbara, California
Huntington Library, Art Collections & Botanial Gardens - San Marino, California
Los Angeles County Museum of Art - Los Angeles, California
San Diego Museum of Art - San Diego, California
San Jose Museum of Art - San Jose, California
Portland Museum of Art - Portland, Oregon
Joslyn Art Museum- Omaha, Nebraska
Sheldon Memorial Art Gallery - Lincoln, Nebraska
University of Montana Fine Arts Museum - Missoula, Montana
Seattle Art Museum - Seattle, Washington
Boulder Museum of Contempory Art - Boulder, Colorado

The Southwest
Fleischer Museum of California and Russian Impressionism - Scottsdale, Arizona
Denver Art Museum - Denver, Colorado
Museum of Art, Brigham Young University - Provo, Utah
Utah Museum of Fine Arts - Salt Lake City, Utah
Phoenix Art Museum - Phoenix, Arizona
The Georgia O'Keeffe Museum - Santa Fe, New Mexico

Canada
National Gallery of Canada - Ottawa, Ontario
The Montreal Museum of Fine Arts - Montreal
Art Gallery of Greater Victoria - Victoria, British Columbia

Other Countries
Tate Gallery - London, England
        -a search on Sargent reveals 47, not all with images
The British Museum - London, England
The National Gallery - London, England
National Portrait Gallery - London, England
The Victoria and Albert Museum - London, England
Royal Academy of Art - London, England
Zorn Museum - Sweden
Palozzo Grazzi - Italy
Borghese Gallery - Rome, Italy
Le Musèe du Louvre - Paris, France
Musee d'Orsay - France
Musèe d'Art Amèricain, Giverny - Giverny, France
Musèe des Augustins - Toulouse, France
Rijksmuseum - Amsterdam
The Israel Museum - Jerusalem
Prado Museum - Madrid, Spain
Museo Thyssen-Bornemisza - Madrid, Spain
National Museum of History - Sofia, Bulgaria
The State Russian Museum - Moscow, Russia
The Hermitage - St. Petersburg, Russia

Western Australian Museum - Australia

Museum Networks & Exhibition Guides
American Alliance of Museums
Art Museum Network
Museum Computer Network
Online Art & Art History
Alexander Palace Russian History
Art Renewal Center
Carol Jackson Fine Art - Virtual Art Museum
AskArt.com - Amazing site with a history of paintings and what they sold for at the top auction houses.