Once you have created a Watermark and assigned it to a gallery, every new photo uploaded into that gallery will inherit the watermark.
In case it is needed, see this link for more details on creating a watermark.
If you set a default Watermark, every new gallery that is created will inherit the default, so every photo that is uploaded into the new gallery will inherit the default.
In case it is needed, see this link for how to set a watermark as the default watermark.
If you upload a photo into an existing gallery, it will inherit the Watermark, or lack thereof, of that gallery.
For more information on creating and applying Watermarks to your photos uploaded to Zenfolio please click here!