As Gary mentioned, Adobe Photoshop has a filter for placing and reading
watermarks.  The version that I have uses the Digimark system.  When you use
this service, you register your facility with Digimarc and subscribe to this
service.  As long as you pay, you are maintained in their database and anyone
who reads a watermark embedded in an image can contact Digimarc and they will
provide your copyright information to them.

http://www.digimarc.com/sterms.html

I can't see how any of this would prevent someone from taking your images, as
this particular watermark is designed to not be visible, your only indication
of it's presence is if you have a program which can read the imbedded
watermark and then indicate that the image is copyrighted.

Mark C. Vang
Freya Ventures
2100 Mediterranean Ave. Suite 15
Virginia Beach, VA 23451
(757) 340-0099