The Library of Congress hosts an online collection of more than 500 political cartoons and caricatures from U.S. History. You can search the collection by keyword and image type. Along with the images you will find links to related resources from the Library of Congress. You could use these public domain works to help students understand the political perspectives surrounding significant political events in U.S. History. A good model for political cartoon-based lesson plans can be found on Cartoons for the Classroom.

Cartoons for the Classroom
is a service of the Association of American Editorial Cartoonists.
Cartoons in the Classroom offers more than one hundred lesson plans
based on editorial cartoons created by the members of the AAEC. Each
lesson plan is available as free pdf download. As you might expect, most
of the lessons deal with current political and economic topics, but you
will also find some lessons that are not time sensitive.

H/T to Tech & Learning Magazine for the LOC link.