public class AutoGreyableImage : Image
{
  static AutoGreyableImage()
  {
    IsEnabledProperty.OverrideMetadata(typeof(AutoGreyableImage),
      new FrameworkPropertyMetadata(true, OnAutoGreyScaleImageIsEnabledPropertyChanged));
  }

  private static void OnAutoGreyScaleImageIsEnabledPropertyChanged(DependencyObject source, DependencyPropertyChangedEventArgs args)
  {
    var image = source as AutoGreyableImage;
    if (image == null || image.Source == null) return;
    var isEnable = Convert.ToBoolean(args.NewValue);
    if (!isEnable)
    {
      var bitmapImage = new BitmapImage(new Uri(image.Source.ToString()));
      image.Source = new FormatConvertedBitmap(bitmapImage, PixelFormats.Gray32Float, null, 0);
      image.OpacityMask = new ImageBrush(bitmapImage);
    }
    else
    {
      var imgSrc = image.Source as FormatConvertedBitmap;
      if (imgSrc != null)
        image.Source = imgSrc.Source;
      image.OpacityMask = null;
    }
  }
}

<Window
  x:Class="wpftest.MainWindow"
  x:Name="wndMain"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:l="clr-namespace:wpftest"
  TextOptions.TextFormattingMode="Display"
  UseLayoutRounding="True"
  SnapsToDevicePixels="True"
  Width="700"
  Height="700">

  <Window.Resources>
    <Style TargetType="{x:Type l:AutoGreyableImage}">
      <Setter Property="Stretch" Value="None"/>
      <Setter Property="HorizontalAlignment" Value="Center"/>
      <Setter Property="VerticalAlignment" Value="Center" />
      <Style.Triggers>
        <Trigger Property="IsEnabled" Value="False">
          <Setter Property="Opacity" Value="0.4"/>
        </Trigger>
      </Style.Triggers>
    </Style>
    <Style TargetType="{x:Type Button}">
      <Setter Property="VerticalAlignment" Value="Center" />
      <Setter Property="HorizontalAlignment" Value="Center" />
      <Setter Property="Padding" Value="5" />
      <Setter Property="Margin" Value="5" />
    </Style>
    <Style TargetType="{x:Type TextBlock}">
      <Setter Property="HorizontalAlignment" Value="Center" />
      <Setter Property="Margin" Value="5" />
    </Style>
  </Window.Resources>
  <StackPanel Orientation="Horizontal">
    <Button>
      <StackPanel>
        <l:AutoGreyableImage Source="/wpftest;component/picture.png"/>
        <TextBlock Text="Enabled button" />
      </StackPanel>
    </Button>
    <Button IsEnabled="False">
      <StackPanel>
        <l:AutoGreyableImage Source="/wpftest;component/picture.png"/>
        <TextBlock Text="Disabled button" />
      </StackPanel>
    </Button>
  </StackPanel>
</Window>