MorgueFileHigh Quality Free Images. The catch... they must be adapted/transformed/modified etc.
Other tabs lead to pay-image sites.
Flickr.com - The CommonsFlickr is an image hosting site. Not all of their images are free to use, but "The Commons" hosts images from multiple collections that ARE public domain.
Flickr.com - Creative Commons SearchFlickr is an image hosting site. Not all of the images there are free, but many users upload content under Creative Commons Licenses which allow people to freely use their images provided they follow certain restrictions.
The Creative CommonsFind out more about how the Creative Commons works, and what the different licences allow.
