Android : Async onPostExecute Not being called, or correctly overridden

I'm very new to Android development, but have deduced that I have not overridden the onPostExecute method correctly (error), but I am lost as to where I've gone wrong. Am I right to assume that onPostExecute is called by the main UI thread and not Async? onPostExecute isn't called at all at the moment, and adding @Override has given me the error..

Basically, I'm just downloading an image. Learning this Async business and Java too.

class ImageReceiver extends AsyncTask<String, Integer, Bitmap>{

private View rootView;

public ImageReceiver(View rootView){
    this.rootView=rootView;
}

@Override
protected Bitmap doInBackground(String... params) {
    System.out.println("got here");
    try {

        URL url = new URL(params[0]);
        HttpURLConnection httpCon = (HttpURLConnection) url.openConnection();

        if(httpCon.getResponseCode() !=200){
            throw new Exception("Failed to connect");
        }


    InputStream is = httpCon.getInputStream();
    Bitmap img = BitmapFactory.decodeStream(is);
    System.out.println(url.toString());

    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    return null;
}
@Override
protected void OnPostExecute(Bitmap img){
    System.out.println("postexecute");

    ImageView im = (ImageView)rootView.findViewById(R.id.incomming_image);
    im.setImageBitmap(img);
}
}

I call the new thead like so in the OnCreate method of the MainActivity

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    View = getWindow().getDecorView().findViewById(android.R.id.content);



    ImageReceiver imageReceiver = new ImageReceiver(View);
    imageReceiver.execute("http://joshuacroft.co.uk/images/killzone_mercenary_2.jpg");

}

Unsure where I can correct, so any help is appreciated.