Another answer is mleroy (because if I remember that the correct WP is based on silverlight, and I often encounter a lack of brush to do such things.) You can do this using the Clip property.
For instance:
<Image Source="blah\yourpicture.jpg" Width="100" Height="100"> <Image.Clip> <EllipseGeometry RadiusX="100" RadiusY="100" Center="100,100"/> </Image.Clip> </Image>
Hope this helps, greetings
Edit add-on: you can also snap your X / Y radius to the width / height of the image for more flexibility on images with dynamic size.
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