Silverlight and ASP.net Bitmap converting issue

Question

my application take snapshots using webcam and save them in a local folder. so i'm using following code to convert my Silverlight Snapshot to a byte array and send it through the Web Service. name of my web service is ImageService.

ImageServiceSoapClient client = new ImageServiceSoapClient();

private void source_CaptureImageCompleted(object sender, CaptureImageCompletedEventArgs e)
        {            
            WriteableBitmap bmp = new WriteableBitmap(320, 240);
            bmp = e.Result;
            byte[] buffer = bmp.ToByteArray();            

            client.SaveImageToLocalAsync(buffer, txtUserName.Text);            
        }

and this is my server side code..

[WebMethod]
        public string SaveImageToLocal(byte[] buffer,string name)
        {                  

            try
            {                
                Bitmap bmp = new Bitmap(320, 240);
                using (MemoryStream stream = new MemoryStream(buffer))
                {
                    bmp = new Bitmap(stream);
                    string saveString = GetFolder() + name + ".jpg";
                    bmp.Save(saveString, System.Drawing.Imaging.ImageFormat.Jpeg);
                }

                return "Servers says : Saved..!!";
            }
            catch (Exception ex)
            {
                return "Server says : " + ex.Message;
            }
        }

so when i run this application, i'm getting an exception saying "Server says : Parameter is not valid"..

can anyone please tell me what's wrong with this code? i spent weeks to solve this problem. please help... :( thanks...