By Popular Demand: Portraits of the Presidents and First Ladies, 1789-Present This site allows one to find pictorial records of the first families from the Library of Congress Prints and Photograph Division. This is well done and first class effort.
From the site:
The Library of Congress has extensive resources for the study of the United States presidents and first ladies. Frequent requests for presidential portraits inspired Prints and Photographs Division staff to compile this ready reference aid of formal and informal pictures in the division's custody. The selected images include at least one likeness of each of the forty-one presidents and most of the first ladies. This presentation inaugurates a series of online illustrated reference aids that will appear under the running title "By Popular Demand".