Most of the photos in this collection are public domain: Pdphoto.org
Wikimedia commons: This vast repository of images and sounds is just like wikipedia. Anyone can be a contributor, and it's FREE for everyone involved. Contributors believe in sharing their work instead of keeping their copyright protections at the forefront. Contributors have donated this material to the public domain--users may search for images and sound and use it based on the contributors stipulations, which often tell you that you can use their image/sound as long as you make your own work freely available as well. If you find an image or sound that you want to use, simply look at the rules for its use before you use, share, or modify it. Most likely, you will be able to whatever you want with it!
Free images to use under creative commons license (not for commercial use--can adapt, modify, use for educational purposes). Some photos are less professional than others; this is a good source for browsing beautiful pictures rather than trying to search for something specific:
NASA Library of Images - NASA should be credited as the source when you use the photos in this great library of high-quality images. Most of the images are in the public domain unless specifically noted--in those cases, you will need to obtain permission to use the photos from the creator of the image (it will be noted).