Copyright marking of photographic works, audio files, etc has recently become a more prevalent concern for those who produce such works. The discussion linked below was undertaken in 1998 and conducted by some of the best encryption minds and audio forensics types of that time. Very little in the discussion topic, problems, solutions, and copyright tools available, has changed since then.
Some years ago (when JP was still here) there was a small thread on how to copyright photos, or protect them in various ways or at least keep them from being stolen or manipulated. But I can't find it now. Most of the ways discussed were easy to defeat anyway, without spending a lot of time trying.
If you are interested in the problems faced by the industry in trying to protect copyrighted works, or if you produce work that you want to copyright, this discussion will be appreciated. Seems to me there is a golden pot waiting for whoever can provide a system without it becoming seriously disruptive of quality.
The discussion below is quite entailed but totally understandable. The excellent presentation of the problems and solutions are most helpful in such a scholarly presentation.
Perhaps a two-cup investment in reading time for some of us