How to change UIButton image in Swift

From your Obc-C code I think you want to set an Image for button so try this way:

let playButton  = UIButton(type: .Custom)
if let image = UIImage(named: "play.png") {
    playButton.setImage(image, forState: .Normal)
}

In Short:

playButton.setImage(UIImage(named: "play.png"), forState: UIControlState.Normal)

For Swift 3:

let playButton  = UIButton(type: .custom)
playButton.setImage(UIImage(named: "play.png"), for: .normal)

in Swift 4, (Xcode 9) example to turn picture of button to On or Off (btnRec):

var bRec:Bool = true

@IBOutlet weak var btnRec: UIButton!
@IBAction func btnRec(_ sender: Any) {
    bRec = !bRec
    if bRec {
        btnRec.setImage(UIImage(named: "MicOn.png"), for: .normal)
    } else {
        btnRec.setImage(UIImage(named: "MicOff.png"), for: .normal)
    }
}

assume that this is your connected UIButton Name like

@IBOutlet var btn_refresh: UIButton!

your can directly place your image in three modes

 // for normal state
btn_refresh.setImage(UIImage(named: "xxx.png"), forState: UIControlState.Normal)
     // for Highlighted state
      btn_refresh.setImage(UIImage(named: "yyy.png"), forState: UIControlState.Highlighted)

        // for Selected state
         btn_refresh.setImage(UIImage(named: "zzzz.png"), forState: UIControlState.Selected)

on your button action

  //MARK: button_refresh action
@IBAction func button_refresh_touchup_inside(sender: UIButton)
{
    //if you set the image on same  UIButton
    sender.setImage(UIImage(named: "newimage.png"), forState: UIControlState.Normal)

    //if you set the image on another  UIButton
       youranotherbuttonName.setImage(UIImage(named: "newimage.png"), forState: UIControlState.Normal)
}

Swift 5

yourButton.setImage(UIImage(named: "BUTTON_FILENAME.png"), for: .normal)