以下是引用片段：
private void btnUploadPicture_Click(object sender, System.EventArgs e)
{
//检查上传文件的格式是否有效
if(this.UploadFile.PostedFile.ContentType.ToLower().IndexOf("image") < 0)
{
Response.Write("上传图片格式无效！");
return;
}

//生成原图
Byte[] oFileByte = new byte[this.UploadFile.PostedFile.ContentLength];
System.IO.Stream oStream = this.UploadFile.PostedFile.InputStream;
System.Drawing.Image oImage = System.Drawing.Image.FromStream(oStream);

int oWidth = oImage.Width; //原图宽度
int oHeight = oImage.Height; //原图高度
int tWidth = 100; //设置缩略图初始宽度
int tHeight = 100; //设置缩略图初始高度

//按比例计算出缩略图的宽度和高度
if(oWidth >= oHeight)
{
tHeight = (int)Math.Floor(Convert.ToDouble(oHeight) * (Convert.ToDouble(tWidth) / Convert.ToDouble(oWidth)));
}
else
{
tWidth = (int)Math.Floor(Convert.ToDouble(oWidth) * (Convert.ToDouble(tHeight) / Convert.ToDouble(oHeight)));
}

//生成缩略原图
Bitmap tImage = new Bitmap(tWidth,tHeight);
Graphics g = Graphics.FromImage(tImage);
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High; //设置高质量插值法
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;//设置高质量,低速度呈现平滑程度
g.Clear(Color.Transparent); //清空画布并以透明背景色填充
g.DrawImage(oImage,new Rectangle(0,0,tWidth,tHeight),new Rectangle(0,0,oWidth,oHeight),GraphicsUnit.Pixel);

string oFullName = Server.MapPath(".") + "/" + "o" + DateTime.Now.ToShortDateString().Replace("-","") + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + ".jpg"; //保存原图的物理路径
string tFullName = Server.MapPath(".") + "/" + "t" + DateTime.Now.ToShortDateString().Replace("-","") + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + ".jpg"; //保存缩略图的物理路径

try
{
//以JPG格式保存图片
oImage.Save(oFullName,System.Drawing.Imaging.ImageFormat.Jpeg);
tImage.Save(tFullName,System.Drawing.Imaging.ImageFormat.Jpeg);
}
catch(Exception ex)
{
throw ex;
}
finally
{
//释放资源
oImage.Dispose();
g.Dispose();
tImage.Dispose();
}
}

来源:网络