Best way to set image source on every page

What is the best way to set the app background programatically on every page?

Currently when my app loads, it retrieves an image from the filesystem and whenever a new page is loaded, I pass the image to the next page through its constructor and manually set the image source on that page to be this image. Example:

(First page)

public partial class Login : PageParent
{
    private Image appBackground = new Image();

    ...

    private async Task LoginExistsAsync()
    {
        appBackgroundBytes = await FileManager.LoadImage(null, "appBackground");
        appBackground.Source = ImageSource.FromStream(() => new System.IO.MemoryStream(appBackgroundBytes));
        GoToMainPage();
    }

    void GoToMainPage()
    {
        Navigation.PushAsync(new MainPage(appBackground));
        Navigation.RemovePage(this);
    }
}

(Second Page)

    public MainPage(Image appBackground)
    {
        InitializeComponent();
        this.appBackground = appBackground;
        background_image.Source = appBackground.Source;
    }

Where background_image is the x:Name of my XML image, which is placed and constrained the same way a background image would be.

The implications of this method are that on every page the image loads slightly later - which looks tacky. There must be a better way of doing this... I have tried implementing control templates, with the hope that I can define it at the start of the app and have it persist until the app is closed, but the control template is re-defined on every instance call, so the same issue would occur.