Alpha of UIBarButtonItem background can not be set to zero

Question

I have an idea to use my own images as BarButton items. I did following:

1. Added UIView to UIToolbar in Interface Builder. View was automatically converted to UIBarButtonItem with UIView contained.

2. Added UIImageView to UIView

3. Set Alpha of the UIView to zero

However, I cannot do anything with alpha on UIBarButtonView and I cannot get rid of background.

Since UIBarButtonItem is not View based, there seems nothing can be done here with out-of-the-box functionality of UIToolbar. Or?