These sites offer free-to-use images. Bing and Google sites will need to be limited to "free to use" to limit the results correctly. To avoid plagiarism make sure you cite/give attribution to the author.
This portal features content that is free to use and reuse. The Library of Congress believes that this content is either in the public domain, has no copyright, or has been cleared by the copyright owner for public use. Each set is based on a theme.
On the lower right of the search page, select Settings. On the Settings page, select the Advanced Search option. Enter the name of the image searching in the "find images with...field. Next, go to usage rights and select Creative Commons Licenses then choose the Advanced Search button.