/// <summary>  
    /// 根据指定尺寸得到按比例缩放的尺寸,返回true表示以更改尺寸  
    /// </summary>  
    /// <param name="picWidth">图片宽度</param>  
    /// <param name="picHeight">图片高度</param>  
    /// <param name="specifiedWidth">指定宽度</param>  
    /// /// <param name="specifiedHeight">指定高度</param>  
    /// <returns>返回true表示以更改尺寸</returns>  
    private bool GetPicZoomSize(ref int picWidth, ref int picHeight, int specifiedWidth, int specifiedHeight)  
    {  
        int sW = 0, sH = 0;  
        Boolean isZoomSize = false;  
        //按比例缩放  
        Size tem\_size = new Size(picWidth, picHeight);  
        if (tem\_size.Width > specifiedWidth || tem\_size.Height > specifiedHeight) //将\*\*改成c#中的或者操作符号  
        {  
            if ((tem\_size.Width \* specifiedHeight) > (tem\_size.Height \* specifiedWidth))  
            {  
                sW = specifiedWidth;  
                sH = (specifiedWidth \* tem\_size.Height) / tem\_size.Width;  
            }  
            else  
            {  
                sH = specifiedHeight;  
                sW = (tem\_size.Width \* specifiedHeight) / tem\_size.Height;  
            }  
            isZoomSize = true;  
        }  
        else  
        {  
            sW = tem\_size.Width;  
            sH = tem\_size.Height;  
        }  
        picHeight = sH;  
        picWidth = sW;  
        return isZoomSize;  
    }  
    /// <summary>  
    /// 无损压缩图片  
    /// </summary>  
    /// <param name="sFile">原图片</param>  
    /// <param name="dFile">压缩后保存位置</param>  
    /// <param name="dHeight">高度</param>  
    /// <param name="dWidth">宽度</param>  
    /// <param name="flag">压缩质量 1-100</param>  
    /// <returns></returns>

    public bool GetPicThumbnail(string sFile, string dFile, int dHeight, int dWidth, int flag)  
    {  
        System.Drawing.Image iSource = System.Drawing.Image.FromFile(sFile);  
        ImageFormat tFormat = iSource.RawFormat;  
        int sW = iSource.Width, sH = iSource.Height;

        GetPicZoomSize(ref sW, ref sH, dWidth, dHeight);

        Bitmap ob = new Bitmap(dWidth, dHeight);  
        Graphics g = Graphics.FromImage(ob);  
        g.Clear(Color.WhiteSmoke);  
        g.CompositingQuality = CompositingQuality.HighQuality;  
        g.SmoothingMode = SmoothingMode.HighQuality;  
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;  
        g.DrawImage(iSource, new Rectangle((dWidth - sW) / 2, (dHeight - sH) / 2, sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);  
        g.Dispose();  
        //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  
            }  
            else  
            {  
                ob.Save(dFile, tFormat);  
            }  
            return true;  
        }  
        catch  
        {  
            return false;  
        }  
        finally  
        {  
            iSource.Dispose();  
            ob.Dispose();

        }  
    }

缩放图片也会改变图片大小