Interesting news! Creative commons just launched a site called CC Search. It's similar to google images, except there are no copyrights attached to the images. Most of them are within public domain or under a creative commons license. Creative Commons licences usually entail attribution to an image, which works fine on websites or social media. It might be out of place in some digital or print marketing materials though.
I've personally gone through the search and looked at the quality of the images. The site touts that it has an archive of over 300 million images. While that is great, you are going to have to dig through an onslaught of subpar photos to find one that you would like to use. In this case, it might be better to use a paid stock photo site - or use a different stockphoto site like pixabay or unsplash.