比较笨的办法：

自定义压缩算法（例如：ICSharpCode.SharpZipLib）预览时，只解压图片文件到临时目录，到临时目录里读取就是了。

生成的代码、是服务端生成

using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;  
/// <summary>
   /// 生成缩略图
   /// </summary>
   /// <param name="originalImagePath">源图路径（物理路径）</param>
   /// <param name="thumbnailPath">缩略图路径（物理路径）</param>
   /// <param name="width">缩略图宽度</param>
   /// <param name="height">缩略图高度</param>
   /// <param name="mode">生成缩略图的方式</param>   
   public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
   {
    Image originalImage = Image.FromFile(originalImagePath);
           
    int towidth = width;
    int toheight = height;
       
    int x = 0;
    int y = 0;
    int ow = originalImage.Width;
    int oh = originalImage.Height;       

    switch (mode)
    {       
     case "HW"://指定高宽缩放（可能变形）               
      break;
     case "W"://指定宽，高按比例                   
      toheight = originalImage.Height * width/originalImage.Width;
      break;
     case "H"://指定高，宽按比例
      towidth = originalImage.Width * height/originalImage.Height;                   
      break;       
     case "Cut"://指定高宽裁减（不变形）               
      if((double)originalImage.Width/(double)originalImage.Height > (double)towidth/(double)toheight)
      {
       oh = originalImage.Height;
       ow = originalImage.Height*towidth/toheight;
       y = 0;
       x = (originalImage.Width - ow)/2;
      }
      else
      {
       ow = originalImage.Width;
       oh = originalImage.Width*height/towidth;
       x = 0;
       y = (originalImage.Height - oh)/2;
      }
      break;                   
     default :
      break;
    }   
           
    //新建一个bmp图片
    Image bitmap = new System.Drawing.Bitmap(towidth,toheight);

    //新建一个画板
    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(originalImage, new Rectangle(0, 0, towidth, toheight),
     new Rectangle(x, y, ow,oh),
     GraphicsUnit.Pixel);

    try
    {           
     //以jpg格式保存缩略图
     bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    catch(System.Exception e)
    {
     throw e;
    }
    finally
    {
     originalImage.Dispose();
     bitmap.Dispose();                       
     g.Dispose();
    }
   }