Wikimedia Commons is an amazing site full of images, sounds and video clips that can be freely used as long as attribution is included. Many languages are supported and there are approximately six million files available on the site. The Welcome page explains more:
Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone, in their own language. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted here. The repository is created and maintained not by paid archivists, but by volunteers. The scope of Commons is set out on the project scope pages.
Unlike traditional media repositories, Wikimedia Commons is free. Everyone is allowed to copy, use and modify any files here freely as long as the source and the authors are credited and as long as users release their copies/improvements under the same freedom to others. The Wikimedia Commons database itself and the texts in it are licensed under the Creative Commons Attribution – Share Alike Licence. The licence conditions of each individual media file can be found on their description pages. More information on re-use can be found at Commons:Reusing content outside Wikimedia and Commons:First steps/Reuse.
This is a great site to use to not only teach students about digital citizenship, but also for them to consider adding to the site by contributing their own images.