How to save image control's image

Question

how to save image control's image into dynamical directory. code is as below.

using asp.net c# web application. 

string MainDir = @"~/StudentImage/";
               String SubDir = DateTime.Now.ToString("dd-MM-yyyy");
               if (!Directory.Exists(Server.MapPath(MainDir)))
               {
                   Directory.CreateDirectory(Server.MapPath(MainDir));
               }
               if (!Directory.Exists(Server.MapPath(MainDir + SubDir)))
               {
                   Directory.CreateDirectory(Server.MapPath(MainDir + SubDir));
               }
               vImage.ImageUrl = Server.MapPath(MainDir + SubDir);
               string imgname = Path.GetFileName(vImage.ImageUrl);
               vImage.ImageUrl=(Server.MapPath(MainDir + imgname));

i have tried but still not working...