The watermark has to overlay the important parts of the image (otherwise, overlaying your own watermark, or simply cropping it out, becomes trivial). It has to

The watermark has to overlay the important parts of the image (otherwise, overlaying your own watermark, or simply cropping it out, becomes trivial). It has to be transparent, because the purpose of having the image available in the first place is that potential customers must be able to evaluate it for their purposes before purchasing a license for it. Slapping a solid color bar across it makes that quite difficult.