The subject of 'Orphan Works' is currently causing a stir in the USA with proposed legislation called the 'Orphan Works Bill'.
It basically proposes that if someone cannot identify the creator of an image following a basic search, then they can call the image an 'Orphan Work' and use it however they want without having to obtain permission and regardless of who owns the copyright.
Though this is proposed US legislation, it could affect all international artists as it could be your images that are described and used as 'Orphan Works' in this way.
The Illustrators' Partnership based in North America is leading the campaigning against this bill and is asking people to write in now and express their concerns with a pro-forma letter they have supplied:
"For international artists and our colleagues overseas, we’ve provided a special link, with a sample letter and instructions as to whom to write."