One of the hottest new fields of research is called digital watermarking. A digital watermark is an information pattern embedded in a picture or sound which:

• doesn’t affect the sound or image
• remains intact after almost any type of editing, copying, or transformation

It’s kind of like a fingerprint for information. The commercial ramifications of watermarking, especially on the world wide web, are huge, but the technology and legality is still very much in flux, and it’s unclear whether or not it’s actually acheivable.