Do it like this:
<Image x:Name="img01" Height="48" VerticalAlignment="Top" Source="Picture.png" Margin="187,25,165,0">
<Image.Triggers>
<EventTrigger RoutedEvent="Window.Loaded">
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetName="img01"
Storyboard.TargetProperty="Opacity" From="0" To="1"
Duration="0:0:1" BeginTime="0:0:0" AutoReverse="True"
RepeatBehavior="Forever"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Image.Triggers>
</Image>