asp.net 中 水印如何制作？

/// <summary>
/// 给图片上水印
/// </summary>
/// <param name="filepath">原图片地址</param>
/// <param name="waterfile">水印图片地址</param>
public void markwater(string filepath, string waterfile)
{
//gif不水印
int i = filepath.LastIndexOf(".");
string ex = filepath.Substring(i, filepath.Length - i);

if (string.Compare(ex, ".gif", true) == 0)
{
return;
}

string modifyimagepath = filepath;//修改的图像路径

int lucencypercent = 25;
Image modifyimage = null;
Image drawedimage = null;
Graphics g = null;
try
{
//建立图形对象
modifyimage = Image.FromFile(modifyimage