I see those a watermarks (which was probably the intention).
In most countries copyright is implied and pretty much all works (art, books, code etc.) is copyrighted from creation. That pretty much is the reason for all the open source/free software, Creative Commons and similar licenses. You would otherwise not be able to reuse those works. Copyright and licenses can be quite a minefield with local variations.
The above is largely the reason why one should differentiate between published != free for use, and also why one should probably consider only using works with known author and compatible license in projects like these.
Side note: Most of
my FlightGear screenshots at Flickr.com are licensed under Creative Commons-Attribution-NonCommercial-ShareAlike (CC BY-NC-SA)