try to test with "Sleak" tool - should get at least 4 images leak:
line 357: rightButton.setEnabled(false);
line 371: leftButton.setEnabled(false);
line 384: upButton.setEnabled(false);
line 397: downButton.setEnabled(false);

• these lines responsible for image leaks. To avoid leaks you should call dispose directly in proper place:
  rightButton.dispose();
  leftButton.dispose();
  upButton.dispose();
  downButton.dispose();