//bmp： 最终要建立的 微缩图 位图对象。
Bitmap bmp = new Bitmap(width, height);

//g: 绘制 bmp Graphics 对象
Graphics g = Graphics.FromImage(bmp);
g.Clear(Color.Transparent);
//为Graphics g 对象 初始化必要参数，很容易理解。
g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

//源图片宽和高
int imgWidth = img.Width;
int imgHeight = img.Height;

//绘制微缩图
g.DrawImage(img, new System.Drawing.Rectangle(0, 0, width, height), new System.Drawing.Rectangle(0, 0, imgWidth, imgHeight)
, GraphicsUnit.Pixel);

ImageFormat format = img.RawFormat;
ImageCodecInfo info = ImageCodecInfo.GetImageEncoders().SingleOrDefault(i => i.FormatID == format.Guid);
EncoderParameter param = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);
EncoderParameters parameters = new EncoderParameters(1);
parameters.Param[0] = param;
img.Dispose();

//保存已生成微缩图，这里将GIF格式转化成png格式。
if (format == ImageFormat.Gif)
{
destPath = destPath.ToLower().Replace(".gif", ".png");
bmp.Save(destPath, ImageFormat.Png);
}
else
{
if (info != null)
{
bmp.Save(destPath, info, parameters);
}
else
{

bmp.Save(destPath, format);
}
}

img.Dispose();
g.Dispose();
bmp.Dispose();
}