How to get session variable in a webmethod()

See more:

ASP.NET

I want to save the user signature as UserNameSig.png.
the code works but Session["UserNameSig"].ToString()
I Suspect that the [WebMethod()] ie having problems with the session variable.
Do anyone know how can solve this.

Thanks

What I have tried:

[WebMethod()]
        public static void UploadPic(string imageData)
        {

            string Pic_Path = HttpContext.Current.Server.MapPath(Session["UserNameSig"].ToString() + ".png");
            using (FileStream fs = new FileStream(Pic_Path, FileMode.Create))
            {
                using (BinaryWriter bw = new BinaryWriter(fs))
                {
                    byte[] data = Convert.FromBase64String(imageData);
                    bw.Write(data);
                    bw.Close();
                }
            }
        }