public override void ViewDidLoad ()
{
    // Perform any additional setup after loading the view

    UIButton button = new UIButton (new System.Drawing.RectangleF(100, 100, 100, 30));
    button.SetBackgroundImage ("imageNeeded", UIControlState.Normal);
    var gestureRecognizer = new UILongPressGestureRecognizer ();
    gestureRecognizer.AddTarget(() => this.ButtonLongPressed(gestureRecognizer));
    button.AddGestureRecognizer(gestureRecognizer);
    this.View.Add (button);
}

public void ButtonLongPressed(UILongPressGestureRecognizer gestureRecognizer)
{
    if (gestureRecognizer.State != UIGestureRecognizerState.Began)
    {
          return;
          // Needed because selector is executed twice, because Long-press gestures are continuous
    }
    // Perform action of opening the dialog to select/take a picture, replacing the ? image with the new image
}