National Portrait Gallery
The National Portrait Gallery gathers together artistic depictions of or by ". . . individuals who have left—and are leaving—their mark on our country and our culture," according to the gallery website.
If you are looking for art relating to early America or politicians, then the exhibits could prove quite useful. While the temptation is to immediately select the web-only exhibitions, the other sections (current and past) all also have well-developed web components. Examples of exhibits worth a look include "Thomas Paine: The Radical Founding Father," "Presidents in Waiting," and "American Origins, 1600-1900."
The museum offers a number of educational resources to accompany their exhibits. These include lesson plans, study guides, teacher's guides, and a quiz game called The Great History Mystery. You can also access short articles on historical figures such as Rosa Parks and Walt Whitman.
If you have something more specific in mind, you can, of course, search the collection.
Face-to-Face podcasts offer an in-depth look at artists, major historical figures, and events as interpreted through a particular artwork.
Of course, visiting a museum with your class is the best way to introduce them to the stories of famous works of art, so give it some thought if you're in the DC area. The section entitled School Programs offers teacher and student online introductory videos and a list of available on-site student programs (with suggested grade levels), while Teacher Programs covers upcoming professional development opportunities.
Think of how frustrating it is to hear stories about friends' co-workers, etc. whom you don't have a face for. If you do nothing else with the collections, consider giving your students the opportunity to visualize the subjects of their studies—the greater the amount of personality apparent in the portraiture, the better. Portraits may just be key to making the past feel real to your students, making the humanity of history readily accessible in their imaginations.