Always cite your sources!
PhotosForClass - Choose an image, then click download. The image you've downloaded automatically adds the citation (credit) under the image. Magic!
Pixabay - Once you find a photo you'd like to use, copy the link to give credit in your bibliography or somewhere in your work.
Creative Commons - Be thoughtful about which of the little boxes you un-check for your search. You do need to cite your source!
Pexels - "Giving credit to the photographer or Pexels is not necessary but always appreciated." (practice good digital citizenship and give credit!)
Better yet, create your own!
Sound & Music:
Royalty Free means you can use it for free without paying the artist, but you still need to say where you got it from.
Jamendo - A community of free, legal and unlimited music published under Creative Commons licenses
Purple Planet Music - Royalty free music
BenSound - Offers about 175 music tracks that you can download for free. You can download and use the music in your video projects for free provided that you credit Bensound for the music.
WikiMedia - Images and sound files. Read the quick part on each sound about the rights to use it. Very clear permissions.