Look for the works of Baroque-era painters, particularly those who painted portraits. To familiarize yourself with this period in art, try the Grove Dictionary of Art's entry on the Baroque. Then look up the artists mentioned in ARTstor.
Significant American portrait painters of the late 18th century include Gilbert Stuart, Benjamin West, Charles Willson Peale, and John Singleton Copley. Look up their works in ARTstor or in the library catalog. Some books from the catalog include America's Old Masters, John Singleton Copley, and John Singleton Copley in America. The links lead to the catalog records for these books, giving you the call number and status (checked in, checked out).