Searches can be limited to Comics and Editorial cartoons in Proquest Historical Newspapers and American Periodicals Series Online - More Search Options, then Document Type.
Associated Press Images Collection is a searchable database of over 700,000 Associated Press photographs, charts and other graphics from the 1840s to the present.
Europhoto offers historical photograph archives of nine European news agencies. Commercial site: copies of the photos are available for sale. Thumbnails available on Europeana (browse by provider).
The Janet A. Ginsburg Chicago Tribune Collection (1880s-1940s) offers over 12,000 images. The Tribune was known for its innovations in presenting visual material.
George Grantham Bain Collection: photographic files of early news picture agency. Worldwide but emphasis on New York City. Largely 1900 to mid-1920s, but some back to 1860s and as late as the 1930s.
The Presidential Elections, 1860-1912: Cartoons from Harper's Weekly and other Leading Journals
Cartoons of the Gilded Age and Progressive Era
1896: Presidential Campaign - Cartoons and Commentary
Political Cartoons and Caricatures (Library of Congress)
Teaching with Documents: Political Cartoons illustrating Progressivism and the Election of 1912