If you're just worried about scanning / printing - there is a "dot pattern" that
prevents a large number of scanners from scanning (and some printers from printing). There is a crack for photoshop to turn off the checking, but that should stop most people from dicking with the pics.
That pattern can hidden fairly well and will still be detected.
EURion constellation - Wikipedia, the free encyclopedia
For high res prints, you should be able to set a moire pattern up that will show up only when printed on a decent printer..