A work is in the public domain if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner. For a detailed chart on Public Domain Terms in the United States see Cornell University Library's, Copyright Information Center. Keep in mind rights vary for different countries and jurisdictions outside of the U.S. Copyright rules are defined by the location of where a work was published and/or distributed.
1. Enter a term into the Public Domain Pictures/Photographs Search
2. Select "All Licenses"
3. Then choose "No restrictions"
4. Click on the desired image and read the accompanying information about it.
If it's in the Public Domain it means it is free to use without violating copyright laws.
More portals for free-to-use and reuse images: