2005.02.01 19:12 "Re: [Tiff] Add watermark.", by Bob Friesenhahn
I don't know much myself but as I understand the requirements, we talk about adding a small picture at specific coordinates. I think the added picture should be somewhat translucent so that the original image will be clearly visible.
This is done via alpha composition. Libtiff does not directly support it. An image processing package like ImageMagick (http://www.imagemagick.org/), the Gimp (http://www.gimp.org/), or Photoshop, is what you will want to use for this purpose. All of these programs are able to read and write TIFF files.