class Program{    static void Main(string[] args)    {
     //G:\zhyue\backup\projects\Test\ConsoleApplication1\img     //var url = "http://seo.jrechina.com/houselist/";     //var res = WebRequestExt.GetData(url);        string img_url = @"D:\Documents\Pictures\壁纸\179 KB.jpg";        string img_savePath = @"G:\zhyue\backup\projects\Test\ConsoleApplication1\img\179 KB.jpg";        string[] files=Directory.GetFiles(@"D:\Documents\Pictures\准备压缩图片");        foreach (var file in files)        {            img_url = file;            img_savePath = @"G:\zhyue\backup\projects\Test\ConsoleApplication1\img\" + Path.GetFileName(img_url);            bool res = ImgHelper.CompressImageWidthTo760(img_url, img_savePath, 120, 200, true);        }        Console.WriteLine("ok");        Console.ReadKey();    }}lic class ImgHelper{    /// 
    /// 无损压缩图片 压缩宽度到指定宽度760 小于指定宽度的判断size是否大于200KB进行质量压缩 宽高不变    /// 
    /// 原图片地址    /// 压缩后保存图片地址    /// 压缩质量（数字越小压缩率越高）1-100    /// 压缩后图片的最大大小 KB    /// 是否是第一次调用    /// 
          public static bool CompressImageWidthTo760(string sFile, string dFile, int flag = 90, int size = 1000, bool sfsc = true)    {        using (Image iSource = Image.FromFile(sFile))        {            ImageFormat tFormat = iSource.RawFormat;            //如果是第一次调用，原始图像的大小小于要压缩的大小，则直接复制文件，并且返回true            FileInfo firstFileInfo = new FileInfo(sFile);            if (sfsc == true && firstFileInfo.Length < size * 1024 && iSource.Width <= 760)            {                firstFileInfo.CopyTo(dFile, true);                return true;            }            if (firstFileInfo.Length > size * 1024 && iSource.Width <= 760)            {
     //超过200KB只压缩质量不压缩宽高                return NewMethodByQuality(sFile, dFile, ref flag, size, tFormat, iSource as Bitmap);            }            //每次压缩一半的宽高比例            double percent = 760.0 / iSource.Width;            int dHeight = (int)Math.Ceiling(percent * iSource.Height);            int dWidth = 760;            int sW = 0, sH = 0;            //按比例缩放            Size tem_size = new Size(iSource.Width, iSource.Height);            if (tem_size.Width > dHeight || tem_size.Width > dWidth)            {                if ((tem_size.Width * dHeight) > (tem_size.Width * dWidth))                {                    sW = dWidth;                    sH = (dWidth * tem_size.Height) / tem_size.Width;                }                else                {                    sH = dHeight;                    sW = (tem_size.Width * dHeight) / tem_size.Height;                }            }            else            {                sW = tem_size.Width;                sH = tem_size.Height;            }            using (Bitmap ob = new Bitmap(dWidth, dHeight))            {                using (Graphics g = Graphics.FromImage(ob))                {                    g.Clear(Color.WhiteSmoke);                    g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;                    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;                    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;                    //g.DrawImage(iSource, new Rectangle((dWidth - sW) / 2, (dHeight - sH) / 2, sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);                    //g.DrawImage(iSource, new Rectangle((int)Math.Ceiling((dWidth - sW) / percent), (int)Math.Ceiling((dHeight - sH) / percent), sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);                    g.DrawImage(iSource, new Rectangle((int)Math.Ceiling((dWidth - sW) / percent), (int)Math.Ceiling((dHeight - sH) / percent), sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);                    g.Dispose();                }                return NewMethodByQuality(sFile, dFile, ref flag, size, tFormat, ob);            }        }    }    /// 
    /// 根据图片质量压缩    /// 
    ///     ///     ///     ///     ///     ///     /// 
          private static bool NewMethodByQuality(string sFile, string dFile, ref int flag, int size, ImageFormat tFormat, Bitmap ob)    {        //以下代码为保存图片时，设置压缩质量        EncoderParameters ep = new EncoderParameters();        long[] qy = new long[1];        qy[0] = flag;//设置压缩的比例1-100        EncoderParameter eParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, qy);        ep.Param[0] = eParam;        try        {            ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();            ImageCodecInfo jpegICIinfo = null;            for (int x = 0; x < arrayICI.Length; x++)            {                if (arrayICI[x].FormatDescription.Equals("JPEG"))                {                    jpegICIinfo = arrayICI[x];                    break;                }            }            if (jpegICIinfo != null)            {                ob.Save(dFile, jpegICIinfo, ep);//dFile是压缩后的新路径                FileInfo fi = new FileInfo(dFile);                if (fi.Length > 1024 * size)                {                    flag = flag - 10;                    CompressImageWidthTo760(sFile, dFile, flag, size, false);                }            }            else            {                ob.Save(dFile, tFormat);            }            return true;        }        catch        {            return false;        }    }}

说明：等比例压缩图片到指定宽度，假如图片小于指定宽度 判断图片大小是否大于指定大小(KB) 如果大于则压缩图片质量 宽高不变

压缩后：