To use a watermark, simply go to the overview of your gallery and click on the "Watermark" slider for the respective album:
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As soon as you activate the slider, you will be asked which watermark you want to use. As soon as you have decided, you will also get a preview of how the watermark will look:

The system then transfers your watermark to the images in the album:

And this is how the whole thing would look afterwards in the gallery: