Wikimedia Commons1 dataset
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-for artists but by volunteers.
Offsite — Over 2 million freely usable media files to which anyone can contribute