The markphotos.py script adds a copyright notice to JPEG pictures as metadata.

Update: This script was moved to my scripts repo on github on 2017-12-28.

It is written in python (version 3) and uses exiftool. This is not a visible notice, but one contained in the file’s metadata. The following in an excerpt from the identify --verbose output of a photos that was marked by markphotos.py:

Image: img_0541.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Class: DirectClass
  Geometry: 3648x2736+0+0
  Resolution: 180x180
  Print size: 20.2667x15.2
  Units: PixelsPerInch
  Type: TrueColor
  ...
  Properties:
     comment: Copyright © 2011 R.F. Smith <rsmith@xs4all.nl>
     ...
     exif:DateTime: 2011:06:08 21:53:32

To speed up the processing of multiple pictures, this script runs in parallel on all the cores that your machine has. If you want to add a visual watermark to a picture, I suggest using the mogrify program from the ImageMagick suite.