Your DataTemplate
is wrong. a StackPanel
is not the appropiate container for that.
Use a DockPanel
instead:
<DataTemplate x:Key="largeIconTemplate">
<DockPanel Width="140" Height="180">
<Image DockPanel.Dock="Top" Margin="6,6,6,9"
Source="{Binding IconName, Converter={x:Static converters:StringPathToBitmapImageConverter.Instance}}"/>
<TextBlock Text="{Binding Description}"
TextTrimming="CharacterEllipsis"
TextWrapping="Wrap"
TextAlignment="Center"
Margin="0,0,0,1" />
</DockPanel>
</DataTemplate>