using System.Drawing.Imaging;
using System.Drawing;
using System.Drawing.Drawing2D;
private void Page_Load(object sender, System.EventArgs e)
{
// Creating Bitmap object.
Bitmap bmp= new Bitmap(Server.MapPath(Request.QueryString["i"]));
// Query string or file name if u want.
Graphics g=Graphics.FromImage(bmp);
g.SmoothingMode = SmoothingMode.AntiAlias ;
// Writing the string u want to add.
g.DrawString(Request.QueryString["t"],
new Font("verdana",12),SystemBrushes.WindowText, 1, 1);
// String can be added from Query string or any string u want to add.
Response.ContentType="image/jpeg";
bmp.Save(Response.OutputStream, bmp.RawFormat) ;
}
There is another code to type a vertical text as watermark.
http://bytescout.com/watermarkingsdk_simple_vertical_text_preset.html 