Under modern international copyright (the Berne Convention and the WTO's TRIPS agreement), almost all remotely recent works automatically fall under copyright, even if there is no copyright notice, even if they are made available free of charge by the author, unless copyright is explicitly waived by the author. (Much older works are a different matter, in that copyright might have expired and/or not have been automatic.) Not adequately tagging an image is one of the things that could get it deleted for Terms of Use violation, along with obscenity and such.