Image:gratisography
There are several sources on the internet where students can get free images without any copyright infringement concerns. Although it is not always required it would be safe to indicate the name of the creator and the platform. It is also always helpful to read “terms of use.”
https://www.flickr.com/search/?text=happy&sort=relevance&license=4%2C5%2C6
http://www.freepixels.com/search.php?action=search&type=plain
