<Rectangle HorizontalAlignment="Left" Width="200" Stroke="#FF000000" StrokeThickness="0" Fill="{DynamicResource MyImageBrush}" >
        <Rectangle.Resources>
            <ImageBrush x:Name="ImgBrush" x:Key="MyImageBrush" ImageSource="tower.jpg" Stretch="UniformToFill" Opacity="0" >
            </ImageBrush>
        </Rectangle.Resources>
        <Rectangle.Triggers>
            <EventTrigger RoutedEvent="Rectangle.Loaded">
                <BeginStoryboard>
                    <Storyboard>
                        <DoubleAnimation x:Name="FadeIn" Storyboard.TargetName="ImgBrush"  Storyboard.TargetProperty="Opacity" From="0" To="100" Duration="0:0:5"/>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </Rectangle.Triggers>
    </Rectangle>

<Rectangle Name="LeftRectangle" Opacity="0" HorizontalAlignment="Left" Width="200" Stroke="#FF000000" StrokeThickness="0" Fill="{DynamicResource MyImageBrush}" >
        <Rectangle.Resources>
            <ImageBrush x:Name="ImgBrush" x:Key="MyImageBrush" ImageSource="tower.jpg" Stretch="UniformToFill" Opacity="100" >
            </ImageBrush>
        </Rectangle.Resources>
        <Rectangle.Triggers>
            <EventTrigger RoutedEvent="Rectangle.Loaded">
                <BeginStoryboard>
                    <Storyboard>
                        <DoubleAnimation x:Name="FadeIn" Storyboard.TargetName="LeftRectangle"  Storyboard.TargetProperty="Opacity" From="0" To="100" Duration="0:4:5"/>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </Rectangle.Triggers>
    </Rectangle>