How to set image for bar button with swift?

I am trying to set an Image for bar button Item for that I have an image like:

with resolution 30 * 30 but while I assign this Image to Bar button Its looks like:

I have assigned image this way :

and If I try this way like making an IBOutlet for the button and set Image programatically form this question and code for that is:

 // Outlet for bar button
 @IBOutlet weak var fbButton: UIBarButtonItem!

// Set Image for bar button
var backImg: UIImage = UIImage(named: "fb.png")!
fbButton.setBackgroundImage(backImg, forState: .Normal, barMetrics: .Default)

but nothing happend with this,

Can anybody tell me what I am doing wrong?

or which is the batter way to do this?

I have achieved that programatically with this code:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        //create a new button
        let button: UIButton = UIButton.buttonWithType(UIButtonType.Custom) as! UIButton
        //set image for button
        button.setImage(UIImage(named: "fb.png"), forState: UIControlState.Normal)
        //add function for button
        button.addTarget(self, action: "fbButtonPressed", forControlEvents: UIControlEvents.TouchUpInside)
        //set frame
        button.frame = CGRectMake(0, 0, 53, 31)

        let barButton = UIBarButtonItem(customView: button)
        //assign button to navigationbar
        self.navigationItem.rightBarButtonItem = barButton
    }

    //This method will call when you press button.
    func fbButtonPressed() {

        println("Share to fb")
    }
}

And result will be:

Same way you can set button for left side too this way:

self.navigationItem.leftBarButtonItem = barButton

And result will be:

And if you want same transaction as navigation controller have when you go back with default back button then you can achieve that with custom back button with this code:

func backButtonPressed(sender:UIButton) {
    navigationController?.popViewControllerAnimated(true)
}

For swift 3.0:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        //create a new button
        let button = UIButton.init(type: .custom)
        //set image for button
        button.setImage(UIImage(named: "fb.png"), for: UIControlState.normal)
        //add function for button
        button.addTarget(self, action: #selector(ViewController.fbButtonPressed), for: UIControlEvents.touchUpInside)
        //set frame
        button.frame = CGRect(x: 0, y: 0, width: 53, height: 51)

        let barButton = UIBarButtonItem(customView: button)
        //assign button to navigationbar
        self.navigationItem.rightBarButtonItem = barButton
    }

    //This method will call when you press button.
    func fbButtonPressed() {

        print("Share to fb")
    }
}

For swift 4.0:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        //create a new button
        let button = UIButton(type: .custom)
        //set image for button
        button.setImage(UIImage(named: "fb.png"), for: .normal)
        //add function for button
        button.addTarget(self, action: #selector(fbButtonPressed), for: .touchUpInside)
        //set frame
        button.frame = CGRect(x: 0, y: 0, width: 53, height: 51)

        let barButton = UIBarButtonItem(customView: button)
        //assign button to navigationbar
        self.navigationItem.rightBarButtonItem = barButton
    }

    //This method will call when you press button.
    @objc func fbButtonPressed() {

        print("Share to fb")
    }
}