ImageView is not loading correctly

In my Activity I have one ImageView

XML Code:

<!-- FOTO LATERAL! -->

                    <ImageView 
                        android:id="@+id/simbolo_raca"
                        android:layout_width="fill_parent"
                        android:layout_height="fill_parent"

                         />
                   <Button 
                        android:id="@+id/button_save"
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:text="Ok. Seguinte Pergunta"
                        android:layout_gravity="right"
                        android:gravity="right"
                        android:paddingRight="30dip"
            </LinearLayout>

This is the Activity:

public class QuestionarioActivityRaca extends Activity{
 ImageView simbolo;
Int position;
Button B_Save;
List<Drawable> List_imagens = new ArrayList<Drawable>();
@Override
public void onCreate(Bundle savedInstanceState) {
simbolo = (ImageView) findViewById(R.id.simbolo_raca);
B_Save = (Button) findViewById(R.id.button_save);
position = 0;
List_imagens.add(getResources().getDrawable(R.drawable.p1));
List_imagens.add(getResources().getDrawable(R.drawable.p2));
List_imagens.add(getResources().getDrawable(R.drawable.p3));
List_imagens.add(getResources().getDrawable(R.drawable.p4));
loadNewImage(position);
B_Save.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
         loadNewImage();
        }
    });
 }
// I Use this method to load the Image
public loadNewImage(int Page)
{    
 new Thread(new Runnable(){
            @Override
            public void run(){
              simbolo.post(new Runnable(){
                    @Override
                    public void run()
                    {
                        simbolo.setImageDrawable(List_imagens.get(Page));
                    }
                });
            }
        }).start();
 }

The first time I call this method, (position = 0) the image doesnt loads. After that, the image is loading correctly.

How I have to do to load the image the first time?

(I can not load the image in the XML using android:src="@drawable/x") because the image could be different anytime.