As Jon mentioned, unless your business is about providing custom photography
and/or graphics, it isn't worth the expense and trouble to truly protect
images.
The best way, at present, is to use digital rights management. There are a
number of companies that provide this technology - all employing different
methods and technologies.
I was VP Technology at a trade association where we specilized in enterprise
content management. Part of what we worked on was the issue of digital
rights management and distribution. For example, how do you prevent a
purchased and downloaded PDF white paper from being forwarded by e-mail by
the purchaser to others for their use when they did not buy the content.
The industry is fledgling and stuggling to survive, but if you do a Google
for DRM or digital rights management, the results page right side "sponsored
links" will lead you to a few organizations. Here is the Google URL:
http://www.google.com/search?hl=en&ie=UTF-8&q=digital+rights+management
HTH
Rich