There are a few ways to know if an image has been ripped.
There are services like
https://www.digimarc.com/ who will have their spiders run the web to discover which of your images appear on other websites. They use an invisible image embedded watermark to make it possible.
If you just want to prove an image is yours, there's Phibit's Icemark
http://www.phibit.com/ which also uses image based invisible watermarks.
But still, the best deterrent is a good old annoying visible copyright watermark. The best program I've found so far is Watermark V2
http://www.pmnet.info/watermark/It's easy to use and efficient.
Thing is, as I was reading recently, copyright of digital data is not implicit in many countries. It all depends if the country where the image was stolen has signed the Bern Convention or not. If it has, copyrights are implicit and don't need to be marked. Otherwise, the © (Your Name), (Date the image was taken) must be written...
I think Leon might be more up to date than I am on the subject...