on page load of default.aspx,
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 protected void Page_Load(object sender, EventArgs e)
        {
            Class1.GrabSingle_Thumb();
        }
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I have created a class file for thumbnail generartion, here goes the class file...
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public class Class1
    {

        public static void GrabSingle_Thumb()
        {
             // this mediamanagerpro is available here... http://www.mediasoftpro.com/downloads.html
            //Declare object of class MediaManagerPro
            MediaManagerPro oMediaManagerPro = new MediaManagerPro();

            //Get the rooth path
            string strRootPath = HttpContext.Current.Server.MapPath(HttpContext.Current.Request.ApplicationPath);

            //set the directory path info for ffmpeg file
            oMediaManagerPro.FFMPEG_Path = HttpContext.Current.Server.MapPath("~\\ffmpeg\\ffmpeg.exe");

            //Set source file info
            oMediaManagerPro.SourceFile_Path = strRootPath + "Videos";
            oMediaManagerPro.SourceFile_Name = "HelloWorld.flv";

            //set output file info
            oMediaManagerPro.OutputFile_Path = strRootPath + "Images";

            //Single Thumb - Properties available in free version
            oMediaManagerPro.Frame_Time = "5";
            oMediaManagerPro.Image_Format = "jpg";
            oMediaManagerPro.Image_Name = "Image_" + DateTime.Now.Ticks;

            ////Single Thumb - Properties available in full version
            //oMediaManagerPro.Width = 320;
            //oMediaManagerPro.Height = 240;

            //grab thumbnails
            MediaInfo oMediaInfo = oMediaManagerPro.Grab_Thumb();

            #region Print Output Information
            StringBuilder strOutputInfo = new StringBuilder();
            strOutputInfo.Append("Image Created = " + oMediaInfo.File_Name + "<br />");
            HttpContext.Current.Response.Write(strOutputInfo);
            #endregion

        }

    }

