Yes the "image pirate" could crop out the watermark or use a graphics editor and remove it. Good point. However, in either case the image is one smaller, or two may show signs of tampering.
One important key factor here though is, that no one has mentioned, is all images made for screen media are 72 ppi, where as any image used in print mediums should be at least 300 ppi. This means that whatever image they do steal, it will need to be 1/3 the size to have a decent rendering on paper.
So when adding the image to the site it would behoove you to not make them too large. Then add the stylized watermark as well. Now they are left with quite a small image to steal.