I wonder if there isn't a startup business model in here somewhere. An artist could sign up for a service that automatically does TinEye or a Google reverse image search for images that they upload, and then flags suspicious uses for the artist to review.

If it's fair use then you can whitelist a particular site. Maybe the software could even do it for you, by learning about the reputation of various sites for this particular activity and automatically assigning risk factors and so on.

Right now, it seems like the only way to catch art thieves is to have the coincidence of someone who's familiar with both your work and the thief's work point it out to you.