private void ThumbnailImage(string oldImagePath, string newImagePath, int newWidth, int newHeight)
{
string originalFilename = oldImagePath;
//生成的高质量图片名称
string strGoodFile = newImagePath;

//从文件取得图片对象
System.Drawing.Image image = System.Drawing.Image.FromFile(originalFilename);

//取得图片大小
System.Drawing.Size size = new Size(newWidth, newHeight);

//新建一个bmp图片
System.Drawing.Image bitmap = new System.Drawing.Bitmap(size.Width, size.Height);

//新建一个画板
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);

//设置高质量插值法
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

//设置高质量,低速度呈现平滑程度
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

//清空一下画布
g.Clear(Color.Transparent);

//在指定位置画图
g.DrawImage(image, new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height), new System.Drawing.Rectangle(0, 0, image.Width, image.Height), System.Drawing.GraphicsUnit.Pixel);
image.Dispose();

//保存高清晰度的缩略图
bitmap.Save(strGoodFile, GetFormat(strGoodFile));

//释放
image.Dispose();
g.Dispose();

}

/// <summary>
/// 得到图片格式
/// </summary>
/// <param name="name">文件名称</param>
/// <returns></returns>
private ImageFormat GetFormat(string name)
{
string ext = name.Substring(name.LastIndexOf(".") + 1);
switch (ext.ToLower())
{
case "jpg":
case "jpeg":
return ImageFormat.Jpeg;
case "bmp":
return ImageFormat.Bmp;
case "png":
return ImageFormat.Png;
case "gif":
return ImageFormat.Gif;
default:
return ImageFormat.Jpeg;
}
}