Just because something is royalty-free does not make it free. Most image banks charge! Many are private pages. There are some creative commons sites as well.
Here are links to some creative common sites:
Free Photo Bank - you must cite this site
Flickr - after you submit a search subject, check the "creative commons, commercial use" check box
Note: Use Google images and similar sites carefully
Images in the public domain are not subject to copyright. They are images that have either expired their copyright time frame, or that have been donated for public use. Here are some links to public domain images:
USFWS National Digital Library - The digital library for the U.S. Fish and Wildlife Services; most images are in the public domain
The British Library - Digitized images from the 17th, 18, and 19th centuries
Smithsonian Institution - Digitized images from some of the Smithsonian Institutions's collections
*Click here to view Cornell's chart on Copyright Term and Public Domain.