The following stock image sites have CC0 licensed images and photographs available for use both with and without attribution, though it's always a good idea to credit your sources. Be sure to check the licenses associated with any images you use.
"This site really does offer you free download of all the pictures in our massive and constantly growing photo gallery. You can use big photo images just as you wish for personal or commercial purposes (see Copyright for details) and there are no watermarks or complicated rules. All we do ask is a link back to our website!"
Dreamstime is a stock photography retailer offering millions of "high resolution Royalty Free stock images". While many are free to use, this site also offers purchase options for licensed content.
FreeDigitalPhotos.net offers small-sized (400 pixels on the longest side) photos and illustrations free of charge. Larger sized versions and unwatermarked versions are available to buy. Attribution is required for all free images.
FreeImages describes itself as "a free pictures website, offering a diverse collection of high-quality images, including photos, vectors, clipart, icons, and PSD files." Any relevant restrictions are visible with the image preview, right next to the Download button.
"We provide a way for developers to gather royalty-free photography that can be used in print, film, TV, Internet or any other type of media both for commercial and personal use."
"Viktor Hanacek is 30-year-old photographer, online entrepreneur and designer living in the Czech Republic. In 2013 he has created this stock photo site called picjumbo with free pictures that are today used by millions of bloggers, designers and marketers all around the world.?
All images available for download from the galleries are completely free, if they are being used within the Terms of Use and Image License rules:
http://www.rgbstock.com/license
http://www.rgbstock.com/termsofuse
The photographs and images are submitted by photographers and artists from around the globe, with hundreds of new images added weekly, many of which have a Creative Commons License. License information is unique to each image, so be sure to check before downloading!