In many ways, you may want to know how pictures in the Public Domain are different from those that are part of the Creative Commons license. First of all, all pictures that can be licensed as part of Creative Commons are owned by someone who decides to allow others the use of their content. Public Domain content, on the other hand does not belong to any single individual or organization. Rather, it belongs to all mankind. It's because of this reason that you cannot establish any rights over any Public Domain content you may use.
Also, you need to be absolutely sure that any picture or content that is supposedly in the public domain is indeed so. There are plenty of ways to ascertain the public domain status of any content:
- If you find the pictures or content on a national government site that expressly indicates a public domain status, then that should be a good indication.
- If you find content on a site that is dubious, it is possible that the content may not be in the public domain.
- While in doubt, ask. You can find the contact information on a site and mail them for more information.
Here are some resources that explore public domain content, especially pictures:
Bing's Image Search has a convenient License filter that lets you search only Public Domain pictures.
The Harvard Law School site has a great explanation along with links to resources for both Public Domain and Creative Commons content.
Bobbe Allen's blog discusses the differences between Public Domain and Creative Commons.
Wikipedia's Public Domain page explains the entire concept in simple words.
Picture Basics: Public Domain Pictures (Glossary Page)