Uhm.. since I never tried, I took a look around and found some answer going like:
I forgot to mention that Digimark can be used to visualy watermark the image, but the program can cost up to $500. The version that comes with photoshop will only make the image look a bit grainy--thus my use of the "other" programs. Take my advice if you put the images on a CD with no obvious water mark or embedded info that makes them impossible to copy or print, your clients will immediately take them to walmart or the nearest CVS pharmacy!
The Digimarc ImageBridge plug-ins enables you to detect, embed, and read Digimarc digital watermarks with many leading image editing applications.
The "pro" version can actually do much more than this apparently, but comes at 500 USD
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