There are many, indeed many sources for free images on the internet but IMHO, these 4 are the best.

Wikimedia Commons
Wikimedia Commons - 3.5 million images that can be used freely under CC
http://commons.wikimedia.org/wiki/Main_Page

Flickr: The Commons
http://www.flickr.com/commons/

Pics4learning
http://www.pics4learning.com/index.php?view=browse
Pics4Learning is a copyright-friendly image library for teachers and students. Pictures have been by students, teachers, and amateur photographers.

Morguefile
http://Morguefile.com
Free images from designers' projects.

This list is incomlete without your suggestions.

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