Public domain ~ Works can be used, adapted, copied manipulated and published without restrictions or permissions. I HQ photography, cartoons to www.pixabay.com (public domain)
Creative Commons ~ Creators set rules for the way their works are to be used works
On flickr.com I submit HQ works for creative commons (where anyone can use without permissions but giving me credit,)
Stock photo websites https://unsplash.com/ http://superfamous.com/ https://picjumbo.com/ https://pixabay.com/ http://imcreator.com/free http://www.gratisography.com/ https://morguefile.com/ http://www.freeimages.com/ http://nos.twnsnd.co/ https://picography.co/ http://getrefe.tumblr.com/ http://jaymantri.com/ http://publicdomainarchive.com/
For a list of descriptions on these photo stock sites go to http://platowebdesign.com/articles/free-stock-photos/
To use steemit stock photos a person would have to sign up and follow the guidelines giving credit of the author and their steemit link.