Convert Powerpoint file into Image file

using Microsoft.Office.Interop.PowerPoint;
using Microsoft.Office.Core;
using System.IO;

try
  {
      FileInfo objfile = new FileInfo(FileUpload1.PostedFile.FileName);
     if (objfile.Extension.Equals(".ppt"))
       {
          ApplicationClass pptApplication = new Microsoft.Office.Interop.PowerPoint.ApplicationClass();
          Presentation pptPresentation = pptApplication.Presentations.Open(objfile.FullName, MsoTriState.msoFalse, MsoTriState.msoFalse, MsoTriState.msoFalse);
          pptPresentation.Export(objfile.FullName, "jpg", Int32.Parse(pptPresentation.SlideMaster.Width.ToString()), Int32.Parse(pptPresentation.SlideMaster.Height.ToString()));
        }
     else
       {
          Response.Write("Please choose PowerPoint files only");
        }
  }
catch (Exception ex)
   { }