请教如何给网站上传的图片加水印？
有很多交友网站，当会员上传了自己的生活照片的时候，网站自动给
每张上传的图片加上 "烙印 "   :如打上自己的网站中文名字和网址等烙印，
"某某网站www.***.com "   等这样的水印，我们的网站是用asp.net   (C#   )编写的程序，请问技术上一般如何实现这个功能呢？



------解决方案--------------------
gdi+写文字上去
------解决方案--------------------
private void Btn_Upload_Click(object sender, System.EventArgs e)
{
if(UploadFile.PostedFile.FileName.Trim()!= " ")
{
//上传文件
string extension = Path.GetExtension(UploadFile.PostedFile.FileName).ToUpper();
string fileName = DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + DateTime.Now.Day.ToString() + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString();
string path = Server.MapPath( ". ") + "/UploadFile/ " + fileName + extension;
UploadFile.PostedFile.SaveAs(path);

//加文字水印，注意，这里的代码和以下加图片水印的代码不能共存
System.Drawing.Image image = System.Drawing.Image.FromFile(path);
Graphics g = Graphics.FromImage(image);
g.DrawImage(image, 0, 0, image.Width, image.Height);
Font f = new Font( "Verdana ", 32);
Brush b = new SolidBrush(Color.White);
string addText = AddText.Value.Trim();
g.DrawString(addText, f, b, 10, 10);
g.Dispose();

//加图片水印
System.Drawing.Image image = System.Drawing.Image.FromFile(path);
System.Drawing.Image copyImage = System.Drawing.Image.FromFile( Server.MapPath( ". ") + "/Alex.gif ");
Graphics g = Graphics.FromImage(image);
g.DrawImage(copyImage, new Rectangle(image.Width-copyImage.Width, image.Height-copyImage.Height, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width, copyImage.Height, GraphicsUnit.Pixel);
g.Dispose();

//保存加水印过后的图片,删除原始图片
string newPath = Server.MapPath( ". ") + "/UploadFile/ " + fileName + "_new " + extension;
image.Save(newPath);
image.Dispose();
if(File.Exists(path))
{
File.Delete(path);
}

Response.Redirect(newPath);
}
}
------解决方案--------------------
用GDI+