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It would be nice if the resizing of the watermark could depend of both the width and height of the picture.

One way would be to refer to the shortest side.
Another way could be to enable the possibility to set both the width and height of the picture as parameters. (A zero value could mean – Don't care!)

That would ensure the watermark don't go outside any of the top, bottom, left or right edge of the picture - both in portrait and landscape pictures.
This is especially useful if you use a centered watermark and particularly if it has considerable width and height – for instance if it is square or round.