Posto qui una versione alternativa al recente post di Igor a dimostrazione che XAML può evitare l'uso del codice,a beneficio di chi non legge i commenti nei blogs... smile_regular

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="WpfApplication9.Window1"
    x:Name="Window"
    Title="Window1"
    Width="640" Height="480">
    <Window.Triggers>
        <EventTrigger RoutedEvent="Window.Loaded">
            <BeginStoryboard  Name="MyStoryboard">
                <Storyboard>
                    <DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="image" Storyboard.TargetProperty="(UIElement.Opacity)">
                        <SplineDoubleKeyFrame KeyTime="00:00:00" Value="0"/>
                        <SplineDoubleKeyFrame KeyTime="00:00:05" Value="1"/>
                    </DoubleAnimationUsingKeyFrames>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
        <EventTrigger RoutedEvent="FrameworkElement.MouseUp" >
            <SetStoryboardSpeedRatio  SpeedRatio="8" BeginStoryboardName="MyStoryboard" />
        </EventTrigger>
    </Window.Triggers>

    <Grid x:Name="LayoutRoot">
        <Image Margin="289,22.75,54.667,211" Source="IMG_0724.JPG" Stretch="Fill" x:Name="image"/>
    </Grid>
</Window>


Per eventuali dubbi sull'argomento: http://msdn2.microsoft.com/en-us/library/ms744905.aspx