Search Flickr for photos Then choose license:
Clipart's slogan is, "Openclipart is the largest community of artists making the best free original clipart for you to use for absolutely any reason."
Search Google Images. Once you search, you can select those images "Labeled for reuse."
All images and videos on Pixabay are released free of copyrights under Creative Commons licensing. You can modify, download, or use them as you please and you do not have to cite these images.