We previously provided a library of stock images, however these were removed in this April 2017 release: http://releases.elucidat.com/2017/04/feature-release-v2-48-00/
We do not specifically recommend or partner with any particular image sources anymore, but some you might like to look at are:
https://unsplash.com/
http://pixabay.com/en/
https://www.pexels.com/
http://elearningart.com/