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You can use a process called steganography if you want a really secure watermark. It embeds the data within the image data, but you can't see it unless you specifically check for it.
You can control how much it changes the image, and even specify to embed the watermark in the contrast/brightness data and so on.
There are free packages out there, but I haven't looked in on the field for quite a while.
Steganos was one software developer with free and professional editions. And you generally get what you pays for!
Hope this helps!
-Pete
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