Use the XAML below with one of your images and the result will be a flipped rectangle.

<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
Title="OpacityMaskBug">

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition/>
        <RowDefinition/>
        <RowDefinition/>
    </Grid.RowDefinitions>
    <Grid.Background>
        <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
            <GradientStop Color="#FF8F1B9C"/>
            <GradientStop Color="#FF076D6F" Offset="1"/>
        </LinearGradientBrush>
    </Grid.Background>
    <Viewbox Grid.Row="1" Opacity="0.95">
        <Rectangle Grid.Row="1" x:Name="TargetRect" Fill="#FFD1F536" Width="256" Height="256">
            <Rectangle.OpacityMask>
                <ImageBrush ImageSource="Images/award_icon.png"/>
            </Rectangle.OpacityMask>
        </Rectangle>
    </Viewbox>
</Grid>

</Page>