How to create rounded button on Navigation Bar? Monotouch.Dialog

Hi, How to create a rounded button or which just displays the background image for Ipad app, using monotouch. I created a UIBarButtonItem with background image, but it displays in rect form. how to show just transparent.

var btn= new UIBarButtonItem(UIImage.FromBundle("ibtn.png"), UIBarButtonItemStyle.Plain, null);
            btn.TintColor = UIColor.Clear;

I have attached the button image that i want to add on navigation bar. right side