The following sites are contain graphics that may be used without obtaining further permsissions from the author.
National Archives and Records Administration Archival Information Locator (NAIL)
Contains many online photographic exhibits, including Picturing The Century containing photographs from every decade in the last 100 years. Usage guidelines are availabe here. You may also consult the Note on Citing Digitized Documents Available at NARA.
Yahoo Gallery
Contains a large searchable collection of images, the majority of which are available for non-commercial use. Many images come from http://corbis.com, and must retain the watermark in order to be used. See the Picture Gallery Help for more information on usage restrictions.
Absolute Web Graphics Archives
A large collection of buttons, icons, lines and clipart for free download and use on web pages. See usage information on the faq page
Library of Congress American Memory
"American Memory is a gateway to rich primary source materials relating to the history and culture of the United States. The site offers more than 7 million digital items from more than 100 historical collections." Some materials may be copyrighted and used only under fair use guidelines. The Library of Congress provides some assistance in determining the copyright status of items, and a Copyright, and Other Restrictions page.
Pierce Art Library (Bowdoin College) - Electronic Resources
The following is a list of websites through which you can find and download graphics. Materials located in this manner may or may not be copyright restricted.