Here's the problem in your code:
protected void Page_Load(object sender, EventArgs e)
{
ApplicationClass pptApplication = new ApplicationClass();
Presentation pptPresentation = pptApplication.Presentations.Open("saifullah.ppt", MsoTriState.msoFalse,MsoTriState.msoFalse, MsoTriState.msoFalse);
String filePath;
filePath = Server.MapPath("/Some/Virtual/Path/on/your/WebSite");
filePath += "\\{0}";
String fileName = "slid.jpg";
String fileNameAndPath = String.Format(filePath, "slid.jpg");
pptPresentation.Slides[1].Export(fileNameAndPath, "jpg", 320, 240);
Image1.ImageUrl = String.Format("/Some/Virtual/Path/on/your/WebSite/{0}", fileName);
}
Regards,
—MRB