Upload Default File Path if field is empty

Question

I am using MVC. I have 'Contact' Table in which I have PhotoID filed. I want save data and if Phoid filed is empty then save default path like '~/Uploads/UploadImages/Guest.PNG' how can I do? my code is below  

 

[HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult CreateContacts(Contact contacts, HttpPostedFileBase ImageFile)/*,TAG2,FLEX1,FLEX2,FLEX3*/
        {

            if (ModelState.IsValid)
            {
                string fileName = Path.GetFileNameWithoutExtension(ImageFile.FileName);
                string extension = Path.GetExtension(ImageFile.FileName);
                fileName = fileName + DateTime.Now.ToString("yymmssfff") + extension;
                contacts.PhotoID = "~/Uploads/UploadImages/" + fileName;
                fileName = Path.Combine(Server.MapPath("~/Uploads/UploadImages/"), fileName);
                ImageFile.SaveAs(fileName);

                using (ESContext db = new ESContext())
                {
                    db.Contacts.Add(contacts);
                    db.SaveChanges();
                    return RedirectToAction("IndexContacts");
                }

                }


            return View(contacts);
        }