WPF image opacity in button style

gistfile1.xml

<Style x:Key="btnCustom" TargetType="{x:Type Button}">
    <Setter Property="Height" Value="50"/>
    <Setter Property="Width" Value="80"/>
    <Setter Property="Margin" Value="5,2,0,5"/>
    <Style.Resources>
        <Style TargetType="Image">
            <Style.Triggers>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Opacity" Value="0.5" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Style.Resources>
</Style>