Use Image as the Grid background

   
        

<Window x:Class="OpacityMaskExample"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title=""
  Height="430" Width="300">
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition Height="Auto" />
      <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Grid Grid.Row="0" Margin="5">
      <Grid.Background>
        <ImageBrush ImageSource="c:image.jpg" />
      </Grid.Background>
      <Ellipse Width="200" Height="200" StrokeThickness="0"
        Fill="Yellow" Margin="20">
        <Ellipse.OpacityMask>
          <RadialGradientBrush GradientOrigin="0.5,0.5"
            Center="0.5,0.5" RadiusX="1" RadiusY="1">
            <GradientStop Offset="0" Color="Transparent" />
            <GradientStop Offset="1" Color="Yellow" />
          </RadialGradientBrush>
        </Ellipse.OpacityMask>
      </Ellipse>
    </Grid>



  </Grid>
</Window>