Hello Mick & Alex!

>There are not a watermarks as it. They are only images. You may make it in
PhotoShop
>using varoius filters. Many of graphic programms allow you add some effects
so you
>paint will look like a drops of wanter. Using a PhotoShop you also can add
slices to a
>picture you have.
>>
>> I've seen various graphic packages that let you add a watermark to a
>> graphic. Even though I understand the reason for having a watermark I
can't
>> imagine how it works ... can some one please explain?

Actually, what he was talking about (I'm assuming) was the copy-protection
watermarking, and not the visible (barely) type of watermark. And since you
mentioned PhotoShop, that's one that can add one via one of the filters.
Now, as to the original question on how it's done, I believe it's by
shifting color at various intervals throught the image (practically
imperceptibly) according to whatever encoding scheme you're using.  You can
probably find more precise (actually, I'm certain you can, I'm not much of a
technical writer) at:

 http://digimarc.com/applications/copyright/copyright_in.html

Their method seems to be the standard from what I've seen. HTH!

 Sean Oelkers
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