[Windows 8] How to create a circle mask image with XAML

The example creates an ImageBrush and sets the ImageSource to an image named people.png, which you must include as a resource in the app. The ImageBrush then paints the area defined by an Ellipse shape.

<Ellipse Height="200" Width="200"> 
    <Ellipse.Fill> 
        <ImageBrush ImageSource="people.png" /> 
    </Ellipse.Fill> 
</Ellipse>

Result:

Reference:
Quickstart: Using brushes (Windows Store apps using C#/VB/C++ and XAML)