<Ellipse Width="100"
         Height="100">
    <Ellipse.Fill>
        <ImageBrush>
            <ImageBrush.ImageSource>
                <BitmapImage UriSource="/YourImage.png" />
            </ImageBrush.ImageSource>
        </ImageBrush>
    </Ellipse.Fill>
</Ellipse>

作为最佳实践,请考虑将DecodePixelWidth和DecodePixelHeight设置为与椭圆相同的大小.