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Although its true that someone with expert Photoshop skills can eliminate a watermark in an hour or so, even if thoughtfully constructed, watermarks still stop most thieves because it's rarely worth spending an hour or more to steal an image, especially if the final product has any telltale visual artifacts shouting, “Theft!”
This past July, at the Computer Vision and Pattern Recognition Conference, Google demonstrated an algorithm capable of automating the removal of watermarks from photos. Google showed that the removal of watermarks from a series of photographs that now takes hours or days, could be done in minutes.