var file = context.Request.Files[0]; 
//从上传图片创建大图
 using (var bigImage = Image.FromStream(file.InputStream))
        {
            //等比创建小图
            using (var smallImage = new Bitmap(200, 200 * bigImage.Height / bigImage.Width))
            {
                //创建画布将大图画入小图
                using (var graphics = Graphics.FromImage(smallImage))
                {
                    graphics.DrawImage(bigImage, 0, 0, smallImage.Width, smallImage.Height);
                }
                //保存
                bigImage.Save(context.Server.MapPath(Guid.NewGuid() + "_big.jpg"));
                smallImage.Save(context.Server.MapPath(Guid.NewGuid() + "_small.jpg"));
            }
        }