.net自带的生成缩略图的方法是Bitmap.GetThumbnailImage这个方法生成的缩略图不够清晰，一般我们会使用 Graphics高质量插值发生成清晰的缩略图，这时候大小会不尽人意，怎么办？我们可以使用jpeg压缩的方法压缩一下图片，这样图片的大小会缩小到压 缩前的十分之一。

下面是完成这项工作的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Drawing;
using System.IO;

namespace ConsoleApplication3_ThumbnailImg
{
    class Program
    {
        const int HEIGHT = 190;
        const int WIDTH = 190;
        static void Main(string[] args)
        {
            string srcImgPath = @"G:\photoes\20060725\Picture 057.jpg";
            string targetFolder = @"E:\Diary\Test\thumbnail";

            SetThumbnail_1(srcImgPath, targetFolder);
            SetThumbnail_2(srcImgPath, targetFolder);
            SetThumbnail_3(srcImgPath, targetFolder);

            Console.Read();
        }

        static void SetThumbnail_1(string srcImgPath, string targetFolder)
        {
            using (Bitmap source = new Bitmap(srcImgPath))
            {
                // return the source image if it's smaller than the designated thumbnail   
                int wi, hi;
                wi = WIDTH;
                hi = HEIGHT;

                // maintain the aspect ratio despite the thumbnail size parameters   
                if (source.Width > source.Height)
                {
                    wi = WIDTH;
                    hi = (int)(source.Height * ((decimal)WIDTH / source.Width));
                }
                else
                {
                    hi = HEIGHT;
                    wi = (int)(source.Width * ((decimal)HEIGHT / source.Height));
                }

                using (Image thumb = source.GetThumbnailImage(wi, hi, null, IntPtr.Zero))
                {
                    string targetPath = Path.Combine(targetFolder, "th_1.jpg");
                    thumb.Save(targetPath);
                }
            }
        }

        static void SetThumbnail_2(string srcImgPath, string targetFolder)
        {
            using (Bitmap source = new Bitmap(srcImgPath))
            {
                // return the source image if it's smaller than the designated thumbnail   
                int wi, hi;
                wi = WIDTH;
                hi = HEIGHT;

                // maintain the aspect ratio despite the thumbnail size parameters   
                if (source.Width > source.Height)
                {
                    wi = WIDTH;
                    hi = (int)(source.Height * ((decimal)WIDTH / source.Width));
                }
                else
                {
                    hi = HEIGHT;
                    wi = (int)(source.Width * ((decimal)HEIGHT / source.Height));
                }

                // original code that creates lousy thumbnails   
                // System.Drawing.Image ret = source.GetThumbnailImage(wi,hi,null,IntPtr.Zero);   
                using (System.Drawing.Bitmap thumb = new Bitmap(wi, hi))
                {
                    using (Graphics g = Graphics.FromImage(thumb))
                    {
                        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                        g.FillRectangle(Brushes.White, 0, 0, wi, hi);
                        g.DrawImage(source, 0, 0, wi, hi);
                    }
                    string targetPath = Path.Combine(targetFolder, "th_2.jpg");
                    thumb.Save(targetPath);
                }

            }
        }

        static void SetThumbnail_3(string srcImgPath, string targetFolder)
        {
            //Configure JPEG Compression Engine   
            System.Drawing.Imaging.EncoderParameters encoderParams = new System.Drawing.Imaging.EncoderParameters();
            long[] quality = new long[1];
            quality[0] = 75;
            System.Drawing.Imaging.EncoderParameter encoderParam = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
            encoderParams.Param[0] = encoderParam;

            System.Drawing.Imaging.ImageCodecInfo[] arrayICI = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();
            System.Drawing.Imaging.ImageCodecInfo jpegICI = null;
            for (int x = 0; x < arrayICI.Length; x++)
            {
                if (arrayICI[x].FormatDescription.Equals("JPEG"))
                {
                    jpegICI = arrayICI[x];
                    break;
                }
            }

            using (Bitmap source = new Bitmap(srcImgPath))
            {
                int wi, hi;
                wi = WIDTH;
                hi = HEIGHT;

                // maintain the aspect ratio despite the thumbnail size parameters   
                if (source.Width > source.Height)
                {
                    wi = WIDTH;
                    hi = (int)(source.Height * ((decimal)WIDTH / source.Width));
                }
                else
                {
                    hi = HEIGHT;
                    wi = (int)(source.Width * ((decimal)HEIGHT / source.Height));
                }

                // original code that creates lousy thumbnails   
                // System.Drawing.Image ret = source.GetThumbnailImage(wi,hi,null,IntPtr.Zero);   
                using (System.Drawing.Bitmap thumb = new Bitmap(wi, hi))
                {
                    using (Graphics g = Graphics.FromImage(thumb))
                    {
                        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                        g.FillRectangle(Brushes.White, 0, 0, wi, hi);
                        g.DrawImage(source, 0, 0, wi, hi);
                    }
                    string targetPath = Path.Combine(targetFolder, "th_3.jpg");
                    thumb.Save(targetPath, jpegICI, encoderParams);
                }

            }

        }
    }
}