I have heard that you I can use all pictures published on US websites whith no concern of copyright. Can anyone confirm this?
Whoever told you that probably doesn't live in the US. I live in the UK and I know that is a lie, copyright is now the responsibility on the EU, UN or NATO depending on your nation.
No, it's not the responsibility of any of those organizations. They don't make laws. It's the responsibility of the person who owns the Copyright. Each nation has their own copyright laws, most based on the Berne Convention.