Citing sources doesn't just mean citing articles and books in your research paper. You also need to cite images and other multimedia that you might include in your own work.
For more information on citing multimedia resources see:
Newbold, Curtis. Can I Use that Picture?. 2014. The Visual Communication Guy. Web. Accessed August 20, 2018
Depending upon when a work was created, it might be in the Public Domain.
Copyright Term and the Public Domain, a guide to copyright duration created by Peter Hirtle at Cornell University, is a comprehensive and useful resource for researching a work's copyright status. You can also use the Copyright Slider from the American Library Association for quick reference.
As a general rule, works registered or published in the U.S. before 1923 are in the public domain.