It would be easy to change that hash by modifying one pixel per image (pick one that is unaffected by the watermark).
azdle: That won't work, because "there's exactly one distinct watermark per image, so 1000 of the same image would have the same watermark."
Kdparker: So subtle alter the image, to trick the watermarker in to thinking the image isn't the same, thus generating more distinct watermarks for the (effectively) same image.