A watermark is a logo or name that is placed on top of an image. Watermarks credit you as the artist of your image and it is very difficult for anyone to remove/change this. The one most used is the copyright notice, best known as the C symbol (©), plus the year the work was published, or the abbreviation Copr.
You should also include the year of completion as well as your name in the watermark. It looks like the image above.