Use a Border for your image and specify the CornerRadius property
<Grid>
<Border Height="200" Width="250" CornerRadius="0,0,50,50">
<Border.Background>
<ImageBrush ImageSource="Images/Desert.jpg" />
</Border.Background>
</Border>
</Grid>
And use the image as a background brush
Here is an example with this xaml. Just change ImageSource

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