Canvas Page

    <Image x:Name="imgBackground" Source="MyImage.png" Stretch="UniformToFill" Width="{Binding Path=ActualWidth, ElementName=container}" Height="{Binding Path=ActualHeight, ElementName=container}" />

Grid Page

		</Grid.RowDefinitions>

		<!-- BACKGROUND IMAGE -->
		<Image x:Name="Assets/imgBackground" Source="MyImage.png" Stretch="UniformToFill" Grid.ColumnSpan="999" Grid.RowSpan="999"/>