here some links where free photos can be found: 1. stock.xchng 2. Freerange Stock 3. Morguefile 4. Pixel Perfect Digital 5. Free Media Goo 6. openphoto.net 7. Flickr 8. Stockvault 9. NASA 10. deviantART’s Stock Images section 11. Everystockphoto 12. Creative Commons Search 13. Dreamstime’s Free Section 14. Fotolia’s Free Section 15. iStockphoto’s Free Images
Are you sure that NASA's images are really free for general use? I'd also be careful of Flickr - seems that one could get into trouble using these images for commercial purposes.
Its looks like the Nasa images are for general use as long as you follow their guidelines - http://www.nasa.gov/audience/formedia/features/MP_Photo_Guidelines.html .
Check their terms first, they are usually not free for commercial websites and you could get into big trouble. NASA images are public domain. Here is the most comprehensive list of public domain images: http://en.wikipedia.org/wiki/Public_domain_image_resources
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