Works for me in swift 4 (square image 40x40)
let imageView = UIImageView()
imageView.frame.size.width = 40
imageView.frame.size.height = 40
imageView.contentMode = .scaleAspectFit
let image = UIImage(named: "YOUR_IMAGE_NAME")
imageView.image = image
navigationItem.titleView = imageView
If you want other measures, try
let imageView = UIImageView(frame: CGRect(x: 0, y: 0, width: 50, height: 100.5)))
imageView.contentMode = .scaleAspectFit
let image = UIImage(named: "YOUR_IMAGE_NAME")
imageView.image = image
navigationItem.titleView = imageView
I hope it serves you. It works for me.