There are many, indeed many sources for free images on the internet but IMHO, these 4 are the best.
Wikimedia Commons - 3.5 million images that can be used freely under CC
Flickr: The Commons
Pics4Learning is a copyright-friendly image library for teachers and students. Pictures have been by students, teachers, and amateur photographers.
Free images from designers' projects.
This list is incomlete without your suggestions.