Это может быть сделано с чем-то вроде этого ...
UIButton *myButton = [UIButton buttonWithType:UIButtonTypeCustom];
[myButton setImage:[UIImage imageNamed:@"myButtonImage"] forState:UIControlStateNormal];
[myButton setTarget: ....];
UIBarButtonItem *barButton = [[UIBarButtonItem alloc] initWithCustomView:myButton];
self.navigationItem.rightBarButtonItem = barButton;
[barButton release];