Cannot set Background Image of Grid Windows Store App through XAML

Question

I am trying to change the background image of my my App but it only accepts default Four images that are placed in Assets folder. ie Logo.png, widelogo.png etc. OR it accepts an PNG with same name as these files in Assets folder.

But I want to attach a BG Image with different name i.e Background.png. I am trying this XAML but it shows nothing:

<Grid.Background>
        <ImageBrush Stretch="Fill" ImageSource="Pics/bg.png"/>
</Grid.Background>

I am new. Plz Guide me