string documentsPath = Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
string localFilename = "downloaded.jpg";
string localPath = Path.Combine(documentsPath, localFilename);
File.WriteAllBytes(localPath, bytes);
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />