1. Open the gallery.
2. Click on CUSTOMIZE > Gallery Settings or if you opened the gallery in the Organizer, SETTINGS.
3. Go to the PHOTO PROTECTION tab and select "Default".
If you check the box "Watermark all photos in this gallery?" it will ONLY apply the watermark to the existing images in that gallery, if you haven't watermarked them, yet. That setting will automatically apply the watermark to the "yet to be uploaded" images during the upload.
You cannot change the watermark on your images this way - you'll need to change/re-apply the watermark in the Organizer in that case. Read more about removing, changing or re-applying the watermark on this page.
4. Save the change.
The default "proof" watermark will not appear on Thumbnails, Small and Medium display sizes. It will only show from Large or above display sizes. Examples: