You can set the Frame's background like so:
rootFrame.Background = Resources("MyBackgroundBrush")
But that will probably not show up since your pages will generally paint over it. Typically you'd set the brush on your Grids. This shouldn't cause flicker and is the most common way backgrounds are set.
<Grid Background="{StaticResource MyBackgroundBrush}"/>
In both cases you'll want to define the image in resources and blank it out in high contrast modes.
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.ThemeDictionaries>
<ResourceDictionary x:Key="Default">
<ImageBrush x:Key="MyBackgroundBrush" Stretch="Fill" ImageSource="Assets/gracie.jpg"/>
</ResourceDictionary>
<ResourceDictionary x:Key="HighContrast">
<SolidColorBrush x:Key="MyBackgroundBrush" Color="{ThemeResource SystemColorWindowColor}" />
</ResourceDictionary>
</ResourceDictionary.ThemeDictionaries>
</ResourceDictionary>
</Application.Resources>
--Rob