Please see my comment to the question. First, too bad you did not tag the application type you are interested in and the language. Secondly, the problem can be too difficult for you. I would suggest you find some ready-to-use solution.
Please see:
http://www.codeproject.com/search.aspx?doctypeid=1&q=%28watermark+OR+%22water+mark%22%29+OR+%28%22.NET%22+OR+%22C%23%22+OR+%22VB.NET%22%29[
^].
At the moment of writing, I saw 22,970 results. If you properly tagged your language(s) and application type, you would have more observable number of search results. Especially if you used Google or Bind. Please try:
http://bit.ly/Swrx9C[
^].
Aha, 363,000,000 search results. I only hope they are ordered by relevance well enough. :-)
—SA