制作缩略图

   /// <summary>
        /// 制作缩略图
        /// </summary>
        /// <param name="fileName">原图路径</param>
        /// <param name="newFileName">新图路径</param>
        /// <param name="maxWidth">最大宽度</param>
        /// <param name="maxHeight">最大高度</param>
        public static void MakeThumbnailImage(string fileName, string newFileName, int maxWidth, int maxHeight)
        {
            Image original = Image.FromFile(fileName);
            Size _newSize = ResizeImage(original.Width, original.Height, maxWidth, maxHeight);
            Image displayImage = new Bitmap(original, _newSize);

            try
            {
                displayImage.Save(newFileName, GetFormat(fileName));
            }
            finally
            {
                original.Dispose();
            }
        }

        /// <summary>
        /// 计算新尺寸
        /// </summary>
        /// <param name="width">原始宽度</param>
        /// <param name="height">原始高度</param>
        /// <param name="maxWidth">最大新宽度</param>
        /// <param name="maxHeight">最大新高度</param>
        /// <returns></returns>
        private static Size ResizeImage(int width, int height, int maxWidth, int maxHeight)
        {
            decimal MAX_WIDTH = (decimal)maxWidth;
            decimal MAX_HEIGHT = (decimal)maxHeight;
            decimal ASPECT_RATIO = MAX_WIDTH / MAX_HEIGHT;

            int newWidth, newHeight;
            decimal originalWidth = (decimal)width;
            decimal originalHeight = (decimal)height;

            if (originalWidth > MAX_WIDTH || originalHeight > MAX_HEIGHT)
            {
                decimal factor;
                // determine the largest factor 
                if (originalWidth / originalHeight > ASPECT_RATIO)
                {
                    factor = originalWidth / MAX_WIDTH;
                    newWidth = Convert.ToInt32(originalWidth / factor);
                    newHeight = Convert.ToInt32(originalHeight / factor);
                }
                else
                {
                    factor = originalHeight / MAX_HEIGHT;
                    newWidth = Convert.ToInt32(originalWidth / factor);
                    newHeight = Convert.ToInt32(originalHeight / factor);
                }
            }
            else
            {
                newWidth = width;
                newHeight = height;
            }
            return new Size(newWidth, newHeight);
        }

来源：http://www.cnblogs.com/dongchunfeng/archive/2011/08/03/2125766.html