Best resources for copyright free images

1. Pixabay.com
   The biggest collection of quality public domain images. No attribution required, no registration for download of high resolution images. Simply great!
2. Clker.com
   An incredible vast library for clip arts, that can either be downloaded as PNG- or as scalable SVG-files. Commercial use allowed, 100% public domain and no registration required.
3. Compfight
   An improved Flickr search: It's not a source of public domain, but you'll find anything you need under a Creative Commons license, even for commercial use.
4. SXC.hu (stock.xchng)
   More than 250.000 royalty free photos about any topic. Free registration and attribution required.
5. Wiki Comons
   10.000.000+ images and diagrams under various Creative Commons licenses; also public domain, but mostly attribution required. Quality reaches from excellent to poor.
6. morgueFile
   About 200.000 photos free for virtually any use. Quality is a tiny issue here, so finding the best shot can be a bit tiresome.
7. Flickr
   No need for explanations: on Flickr you'll find anything. Use extended search and select the option "Creative Commons" to get what you desire.
8. Deviantart
   A strong  photographer and designer community. All image are under user selected licenses. It's a stunning resource, but be careful and always read the requirements for each image!
9. PublicDomainPictures.net
   Excellent collection of public domain photos, including some extremely high resolution photos, that require payment for downloading (up to 100 MB per image!).
10. gimp-savvy.com
    Link collection of official US agency image libraries: NASA, FWS, NOAA, etc. Source of millions and millions of photos.

Look here, if the above mentioned sources are exhausted. The following websites are rather small, but still, here and there you might find just what you are looking for.

1. aarinfreephoto.com
   Ca. 1.000 photos. Attribution required.
2. abstractinfluence.com
   Abstract images
3. adigitaldreamer.com
   600+ photos mainly about nature.
4. afflict.net
   Various textures: metal, wood, ...
5. amazingtextures.com
   Again: 2.000+ textures.
6. amgmedia.com
   Rather low resolution and attribution required.
7. amygdela.com
   Great for landscapes.
8. ancestryimages.com
   Maps und portraits from 17th to 19th century.
9. artfavor.com
   Non-commercial use of clip arts as well as sounds and fonts.
10. backgroundsarchive.com
    Background images, mostly in low resolution, some in HD.
11. bigfoto.com
    Travel pics from all over the world. Rather low quality.
12. burningwell.org
    Public Domain Images of different quality. Tedious navigation.
13. cepolina.com
    5.000+ image. Attribution required.
14. creativity103.com
    1.500 abstract or macro shots. Attribution!
15. davidniblack.com
    Ca. 500 Bilder 1600×1200px on various topics.
16. digitalgallery.nypl.org
    600.000+ images of the New York Public Library.
17. fotodatenbank.com
    1.600 images, attribution obligatory.
18. free-stockphotos.com
    Great shots for commercial use. Attribution!