1- Crie uma nova página C# com o nome.  thumbnails.aspx.

2- No Page load da página adicione.

        string FileName = Request.QueryString["img"];
        int MaxH = Convert.ToInt32(Request.QueryString["maxH"]);
        int MaxW = Convert.ToInt32(Request.QueryString["maxW"]);  

        Image OriImg = Image.FromFile(Server.MapPath(FileName));
        if (OriImg.Width > MaxW)
        {
            MaxH = Convert.ToInt32(OriImg.Height*MaxW)/OriImg.Width;
        }
        else
        {
            if (OriImg.Height > MaxH)
            {
                MaxW = Convert.ToInt32(OriImg.Width*MaxW)/OriImg.Height;
            }
            else
            {
                MaxW = OriImg.Width;
                MaxH = OriImg.Height;
            }
        }  

        Image thumbnail = new Bitmap(MaxW, MaxH, OriImg.PixelFormat);
        Graphics graphic = Graphics.FromImage(thumbnail);
        var Params = new EncoderParameters(1);
        Params.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
        graphic.InterpolationMode = InterpolationMode.HighQualityBilinear;
        graphic.SmoothingMode = SmoothingMode.HighQuality;
        graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
        graphic.CompositingQuality = CompositingQuality.HighQuality;
        graphic.DrawImage(OriImg, 0, 0, MaxW, MaxH);
 

        var MS = new MemoryStream();
        thumbnail.Save(MS, ImageFormat.Png);
        byte[] MsAr = MS.ToArray();
        Response.ContentType = "image/Png";
        Response.BinaryWrite(MsAr);  

        if (MS != null)
        {
            MS.Close();
            MS.Dispose();
        }  

        if (OriImg != null)
        {
            OriImg.Dispose();
            OriImg = null;
        }

4 - Para uso:
<asp:Image ID="Image1" runat="server" ImageUrl="~/thumbnails.aspx?img=suaImagem.jpg&MaxW=200&MaxH=200" />