The Smithsonian is a wonderful resource for teachers wishing to integrate primary source documents and other artifacts into history instruction. Their online sites include rich collections of visual resources for student analysis. In addition, they offer training for teachers interested in using their resources. One such training is "Teaching with Documents and Works of Art: An Integrated Approach" taught by LeeAnn Potter and Elizabeth K. Eder.
In particular, students of arts in U.S. history may enjoy browsing the National Archives and Records Administration, Smithsonian American Art Museum (particularly see "Teachers and Students" which links to interactive features such as "Campfire Stories with George Catlin," a timeline of Native American history described through art and primary sources), and National Portrait Gallery sites.