Может быть, это может помочь вам:
string strPhoto = Request.Form["imageData"]; //Get the image from flash file
byte[] photo = Convert.FromBase64String(strPhoto);
FileStream fs = new FileStream("PathInTheServer", FileMode.OpenOrCreate, FileAccess.Write);
BinaryWriter br = new BinaryWriter(fs);
br.Write(photo);
br.Flush();
br.Close();
fs.Close();