Here is a selection of sites you can use to source copyright cleared images - sometimes this is where the author has given permission for the image/film/music to be freely used, sometimes it's where the copyright has expired on the resource:
- For images there's Pixabay or Pexels or Unsplash
- Don't forget Flickr Commons, but make sure you're using The Commons and not just normal Flickr
- Use Creative Commons to find reusable images, videos, music and other media content
- Also for music you could use Free Music Archive
- If you wanted to use film clips you could also use the Internet Archive
These sites will give you instructions on how to cite the sources you use (see the image and citation below). Many images are cleared for use provided you 'attribute' the author. Similarly you can contribute your images/music/clips to these sites if you're happy for others to use them freely.
CC-BY-SA-3.0 Nina Paley (2010) Mimi & Eunice, “Thief”.