Most of the images used in this site are found at the Creative Commons section on Flickr. You can see the user agreements on each file I'll link you to:

1. http://www.flickr.com/photos/booberrystudio/3603433508/sizes/o/
2. http://www.flickr.com/photos/24471966@N04/2967303557/sizes/l/in/set-72157606058068182/
3. http://www.flickr.com/photos/24471966@N04/2851251413/sizes/l/
4. http://www.flickr.com/photos/pomo/2806544216/sizes/l/