using System.IO;
using System.Drawing.Imaging;

private void Button1_ServerClick(object sender, System.EventArgs e)
{
Graphics g=null;
System.Drawing.Image upimage=null;
System.Drawing.Image thumimg=null;
System.Drawing.Image simage=null;
Bitmap outputfile=null;
try
{
string extension = Path.GetExtension(File1.PostedFile.FileName).ToUpper();
string filename = DateTime.Now.ToString("yyyyMMddhhmmss");
string smallpath = Server.MapPath(".")+"/smallimg/";
string bigpath = Server.MapPath(".")+"/bigimg/";
int width,height,newwidth,newheight;

System.Drawing.Image.GetThumbnailImageAbort callb =new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback); 
if(!Directory.Exists(smallpath))
Directory.CreateDirectory(smallpath);
if(!Directory.Exists(bigpath))
Directory.CreateDirectory(bigpath);

Stream upimgfile = File1.PostedFile.InputStream;
string simagefile = Server.MapPath("a8logo.jpg"); //要加水印的文件
simage=System.Drawing.Image.FromFile(simagefile);
upimage= System.Drawing.Image.FromStream(upimgfile); //上传的图片

width = upimage.Width;
height = upimage.Height;
if(width>height)
{
newwidth=200;
newheight =(int)((double)height/(double)width * (double)newwidth);
}
else
{
newheight=200;
newwidth=(int)((double)width/(double)height * (double)newheight); 
} 
thumimg = upimage.GetThumbnailImage(newwidth,newheight,callb,IntPtr.Zero);
outputfile=new Bitmap(upimage);
g=Graphics.FromImage(outputfile);
g.DrawImage(simage,new Rectangle(upimage.Width-simage.Width,upimage.Height-simage.Height,upimage.Width,upimage.Height),0,0,upimage.Width,upimage.Height,GraphicsUnit.Pixel);

string newpath = bigpath + filename + extension; //原始图路径
string thumpath = smallpath + filename + extension; //缩略图路径
outputfile.Save(newpath);
thumimg.Save(thumpath);
outputfile.Dispose();

}
catch(Exception ex)
{
throw ex;
}
finally
{
if(g!=null)
g.Dispose();
if(thumimg!=null)
thumimg.Dispose();
if(upimage!=null)
upimage.Dispose();
if(simage!=null)
simage.Dispose();
}
}
public bool ThumbnailCallback()
{
return false;
}

文章来源：网络