American National Biography (ANB) is the first place to look for detailed yet brief biographies of deceased Americans. In addition to the biography itself, the bibliography is useful for directing you to the best first-stop resources and will tell you if and where the person's personal papers and archives exist. You'll also be directed to any reliable online resouces. ANB may not be as useful for earlier periods of American history, especially women and minorities. Click on the box below to go to ANB.
The original of this painting is at the National Museum of American Art in Washington, D.C.
Since many biographies focus on an individual's influence within his or her profession, don't overlook scholarly encyclopedias such as Oxford Art Online or scholarly reference works on a specific field, like Scribner's Writers. The information in these sources will likely provide you with a good evaluation of influences on or by your person and will lead you to more specific sources in disciplines you might not be familiar with such as art, literature, science, etc.