ItemsControl
не отслеживает SelectedItem
Если вы хотите такое поведение, я бы предложил перезаписать стили и шаблоны для ListBox
<ListBox Name="itemsCtrl" HorizontalContentAlignment="Center" ItemsSource="{Binding Path=MovieCollection, Mode=TwoWay}">
<ListBox.Resources>
<!-- Set SelectedItem Background here -->
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Yellow"/>
</ListBox.Resources>
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<UniformGrid Columns="5" />
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Vertical" HorizontalAlignment="Center">
<Image Style="{StaticResource ImageHover}" Margin="0,5,0,0" RenderOptions.BitmapScalingMode="HighQuality" SnapsToDevicePixels="True" HorizontalAlignment="Center" Source="{Binding Path=Poster}" Height="150" />
<TextBlock Name="txtTitle" FontSize="10" HorizontalAlignment="Center" TextAlignment="Justify" Text="{Binding Title}" TextWrapping="Wrap" MaxWidth="110" />
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>