In case anyone needs Swift code for accepted answer:
let infoImage = UIImage(named: "my-icon-32.png")
let imgWidth = infoImage?.size.width
let imgHeight = infoImage?.size.height
let button:UIButton = UIButton(frame: CGRect(x: 0,y: 0,width: imgWidth!, height: imgHeight!))
button.setBackgroundImage(infoImage, forState: .Normal)
button.addTarget(self, action: Selector("openInfo"), forControlEvents: UIControlEvents.TouchUpInside)
self.navigationItem.rightBarButtonItem = UIBarButtonItem(customView: button)
P.S. Gurpreet Singh answer is working only for transparent PNG also I have to set button tintcolor other than clearColor.