Xamarin Forms iOS Navigation Bar Image

I am attempting to set the Navigation Bar Background Image using the NavigationPage.SetTitleIcon(this, "image.png"). However, what appears is the image centered in the Navigation Bar with white space on each side (even though the image is 320 x 44).

My code is:

public partial class frmLogin : ContentPage
    {
        public frmLogin()
        {
            InitializeComponent();
            NavigationPage.SetTitleIcon(this, "image.png");
        }
    }

This is called from my Main.cs via MainPage = new NavigationPage(new frmLogin());

This shouldn't be tough but I have been unable to find any example / answers.