"Public Domain" does NOT mean the same thing as "available on the Internet." "Public Domain" is a very specific legal term that refers to something that is not copyrighted because 1. The creator decided to put it in the public domain. These days, when someone does this, they usually use a Creative Commons license. 2. The creator has allowed the copyrightt to expire and not submitted any paperwork to maintain their copyrights.
Unless there is a notice that says otherwise, things on the Internet are, by default, copyrighted.
When a web site offers up photos or clip are "for free," that only means they aren't going to charge you money for them. That does not mean they are not copyrighted.
The best place to find information about copyright on a web site are in the "Terms of Service," "Terms and Conditions," "Terms of Use," or other parts of the web site that talk about criteria that users of the site must adhere to.