Xamarin.Forms Page BackgroundImage property
If you need a solution that allows you to change the AspectRatio and adjust the image you can use this:
XAML:
<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Pages.PhotoPage">
<Grid >
<Image Source="background.png" Aspect="AspectFit" />
<!-- Place here the rest of the layout for the page. -->
</Grid >
</ContentPage>
If I'm not mistaken you can't share resources. you must put the image in Platform Specific folder and than use BackgroundImage = "image.png"
without Images/
EDIT:
It seems I was partly mistaken.
It is possible to share images by embedding them instead of having multiple copies for different Platforms: https://developer.xamarin.com/guides/xamarin-forms/working-with/images/#Embedded_Images