Edit or Remove the Watermark for Existing Images in a Collection:
Go to the Collection and photo management view, select one or more images you’d like to make this change to, and select the “Watermark” option under the More drop down menu.
Select the new watermark (or No Watermark to remove a watermark) you’d like to apply to these photos. Note that it will take some time for these photos will be reprocessed with the new watermark, these photos will be unavailable to view in the gallery during this time.
You can also choose to Apply this change to all photos in this collection. This will include all the photos in each of the sets in this collection.
How do I watermark my photos?