check this

    // Get the path of the original Image
     string displayedImg = Server.MapPath("~") + "/Testing.jpg";

    // Get the path of the Thumb folder
    string displayedImgThumb = Server.MapPath("~") + "/Thumb/";
    
    // Get the original image file name
    string imgFileName = System.IO.Path.GetFileName(displayedImg);
 
    // Load original image
    System.Drawing.Image myimg = System.Drawing.Image.FromFile(displayedImg);

    // Get the thumbnail 100 X 100 px
    myimg = myimg.GetThumbnailImage(100, 100, null, IntPtr.Zero);
    
   // Save the new thumbnail image
   myimg.Save(displayedImgThumb + imgFileName, myimg.RawFormat);

Hope this helps