Use DoubleAnimation to animate the Opacity
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<Grid x:Name="LayoutRoot" Background="White">
<Image x:Name="myImage" Source="http://server.com/man.png">
<Image.Triggers>
<EventTrigger RoutedEvent="Image.Loaded">
<BeginStoryboard>
<Storyboard x:Name="myStoryboard">
<DoubleAnimation Duration="0:0:1"
Storyboard.TargetName="myImage"
Storyboard.TargetProperty="Opacity"
From="0" To="1" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Image.Triggers>
</Image>
</Grid>
</UserControl>
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