Free photos of eagles are available for download from Wikimedia.org, PhotoPin.com and the Creative Commons section of Flickr.com. The photos on these sources are free of charge and free to reuse in publications, and the site displays details of licensing permissions for each photo.
Wikimedia.org includes only material released under Creative Commons licensing, a copyright provision that content creators can apply to their work in order to allow others to use it while retaining their own rights. Depending on the particular license, restrictions may apply to usage such as requiring attribution or limiting reuse to non-commercial purposes. Wikimedia.org clearly labels the licensing of each image in its collection eagle images, which includes a dozen different species.
Flickr.com hosts a collection of user-created images, including a Creative Commons section for images that utilize that licensing. Flickr's search function allows users to search with keywords and sort by license to include commercial use, non-commercial use or public domain.
PhotoPin.com is another photo curator that includes only Creative Commons photos in its collection. Its search function allows sorting by commercial or non-commercial licensing restrictions. On its image download pages, PhotoPin.com includes ready-made HTML code that provides attribution of the image and links to the appropriate license.