这个C#类可以生成各种形式的缩略图，可以自动保持图片比例缩略，可以根据百分比获得图片尺寸等

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
  
namespace HtmlSnap
{
    public static class ImageHelper
    {
   
        /// <summary>
        /// 获取缩略图
        /// </summary>
        /// <param name="image"></param>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <returns></returns>
        public static Image GetThumbnailImage(Image image, int width, int height)
        {
            if (image == null || width < 1 || height < 1)
                return null;
  
            // 新建一个bmp图片
            //
            Image bitmap = new System.Drawing.Bitmap(width, height);
  
            // 新建一个画板
            //
            using (Graphics g = System.Drawing.Graphics.FromImage(bitmap))
            {
  
                // 设置高质量插值法
                //
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
  
                // 设置高质量,低速度呈现平滑程度
                //
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                  
                // 高质量、低速度复合
                //
                g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
  
                // 清空画布并以透明背景色填充
                //
                g.Clear(Color.Transparent);
  
                // 在指定位置并且按指定大小绘制原图片的指定部分
                //
                g.DrawImage(image, new Rectangle(0, 0, width, height),
                    new Rectangle(0, 0, image.Width, image.Height),
                    GraphicsUnit.Pixel);
  
                return bitmap;
            }
        }
  
        /// <summary>
        /// 生成缩略图，并保持纵横比
        /// </summary>
        /// <param name="image"></param>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <returns>生成缩略图后对象</returns>
        public static Image GetThumbnailImageKeepRatio(Image image, int width, int height)
        {
            Size imageSize = GetImageSize(image, width, height);
            return GetThumbnailImage(image, imageSize.Width, imageSize.Height);
        }
  
        /// <summary>
        /// 根据百分比获取图片的尺寸
        /// </summary>
        /// <param name="picture"></param>
        /// <param name="percent"></param>
        /// <returns></returns>
        public static Size GetImageSize(Image picture, int percent)
        {
            if (picture == null || percent < 1)
                return Size.Empty;
  
            int width = picture.Width * percent / 100;
            int height = picture.Height * percent / 100;
  
            return GetImageSize(picture, width, height);
        }
  
        /// <summary>
        /// 根据设定的大小返回图片的大小，考虑图片长宽的比例问题
        /// </summary>
        /// <param name="picture"></param>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <returns></returns>
        public static Size GetImageSize(Image picture, int width, int height)
        {
            if (picture == null || width < 1 || height < 1)
                return Size.Empty;
  
            Size imageSize;
  
            imageSize = new Size(width, height);
  
            double heightRatio = (double)picture.Height / picture.Width;
            double widthRatio = (double)picture.Width / picture.Height;
  
            int desiredHeight = imageSize.Height;
            int desiredWidth = imageSize.Width;
  
  
            imageSize.Height = desiredHeight;
            if (widthRatio > 0)
                imageSize.Width = Convert.ToInt32(imageSize.Height * widthRatio);
  
            if (imageSize.Width > desiredWidth)
            {
                imageSize.Width = desiredWidth;
                imageSize.Height = Convert.ToInt32(imageSize.Width * heightRatio);
            }
  
            return imageSize;
        }
  
  
        /// <summary>
        /// 获取图像编码解码器的所有相关信息
        /// </summary>
        /// <param name="mimeType">包含编码解码器的多用途网际邮件扩充协议 (MIME) 类型的字符串</param>
        /// <returns>返回图像编码解码器的所有相关信息</returns>
        public static ImageCodecInfo GetCodecInfo(string mimeType)
        {
            ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
            foreach (ImageCodecInfo ici in CodecInfo)
            {
                if (ici.MimeType == mimeType) return ici;
            }
            return null;
        }
  
        public static ImageCodecInfo GetImageCodecInfo(ImageFormat format)
        {
            ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
  
            foreach (ImageCodecInfo icf in encoders)
            {
                if (icf.FormatID == format.Guid)
                {
                    return icf;
                }
            }
  
            return null;
        }
  
        public static void SaveImage(Image image, string savePath, ImageFormat format)
        {
            SaveImage(image, savePath, GetImageCodecInfo(format));
        }
  
        /// <summary>
        /// 高质量保存图片
        /// </summary>
        /// <param name="image"></param>
        /// <param name="savePath"></param>
        /// <param name="ici"></param>
        private static void SaveImage(Image image, string savePath, ImageCodecInfo ici)
        {
            // 设置 原图片 对象的 EncoderParameters 对象
            //
            EncoderParameters parms = new EncoderParameters(1);
            EncoderParameter parm = new EncoderParameter(Encoder.Quality, ((long)95));
            parms.Param[0] = parm;
  
            image.Save(savePath, ici, parms);
            parms.Dispose();
        }
  
    }
}