A watermark is a recognizable image or pattern in paper that appears lighter when viewed by transmitted light (or darker when viewed by reflected light, atop a dark background).
Under Linux you can use ImageMagick software to impose digital watermark (it is a technique which allows an individual to add hidden copyright notices or other verification messages to digital audio, video, or image signals and documents.) for copyright information or to put your site url. For example, consider following image:
To add watermark you need to use composite command which composite images together including watermark image.
$ composite -compose bumpmap -gravity southeast watermark.jpg input.jpg output.jpg