WrapPanel doesn't wrap in WPF ListView

I am using a ListView with an ItemTemplate like this:

<Window.Resources>
    <DataTemplate x:Key="ItemTemplate">
        <WrapPanel Orientation="Horizontal">
            <Image Width="50" Height="50" Stretch="Fill" Source="{Binding Cover}"/>
            <Label Content="{Binding Title}" />
        </WrapPanel>
    </DataTemplate>
</Window.Resources>

But the Covers do not fill the screen like windows explorer windows.

How do I do this? They just get stacked vertically in my version.

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