Earlier this week, the New York Public Library announced that it has made nearly 200,000 public domain images available as free high-resolution downloads in its NYPL Digital Collections. Previously, the items had been available to search and view, but could be downloaded only in low-resolution formats; high-resolution downloads were available by request and required a processing fee.
Google Image Search and Flickr provide easy access to millions of images, the image quality may be lower than needed for an audiovisual project, and there are added concerns about copyright and fair use. (Tip: use Google Advanced Image Search or Flickr's post-search filters to limit results not only to the appropriate size, but also to clarify usage rights to locate public domain or Creative Commons license images.)
The Duke University Libraries also offer several subscription databases with images, which could enhance presentations or other educational research. AP Images includes millions of photographs from the Associated Press files over the last century. Duke researchers may also access the Art Museum Image Gallery, which contains nearly 100,000 images from 3000 B.C. to the present day. If using subscription resources, be sure to consult any associated use restrictions on the items you'd like to download.
For more information about copyright law and permissions, the Medical Center Library at Duke has a helpful guide to Copyright and Education, which covers such topics as obtaining permissions and fair use. This guide also links to an online treatise by Duke's Scholarly Communications Officer (and Law School LARW research instructor) Kevin L. Smith, Owning and Using Scholarship: An IP Handbook for Teachers and Researchers (2014). Chapter 4, in particular, discusses "Using Copyrighted Works in Scholarship" and contains the important research reminder that "The Internet is NOT the Public Domain."
For help with locating public domain images, finding library materials on copyright law, or anything else, be sure to Ask a Librarian.