Surface processing

Okey. I''ve searched around a bit for information how to do some image processing on a surface, but without any luck. What I wan''t to is when I''ve captured my buffer for a screenshot I want to place a watermark on it before I write it to a bmp. So. Do anybody know where to find a tutorial or some kind of info on how to do this? Or maybe just post which DX function I should use. My code is in DX9. Thanks in advanced