In moment Piwigo uses png files for watermark but this creates many problems if pictures colour differs from light to dark. But there is much better way - creating watermark from pure text and with help Imagick. I personally have used this in my site many times.
Look at here and you understand why this is so much better http://valokuva.org/?p=59
(Choosing watermark color based on the background luminosity)
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Which means more computation whereas you could have a png with a black text and white shadow
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+1 flop25
I think it's much simpler to have a PNG with transparent background, white text and black drop shadow (with no horizontal or vertical offset)
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No doubt png is more simple way if I consider computing resource but choosing watermark colour based on the background just looks so much better. Also it is much easier solution to end-user!
Maybe it is reasonable to let end-user choose btw png and simple text.
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