WPF: Image bigger than parent container

I am doint my first WPF-Project with the MVVM-Model. I have got two views that i want to look like that:

But sadly the images do not shrink to parent size, they stay in original size:

Views.PictureList

<UserControl x:Class="BIF.Views.PictureList"
         xmlns:view="clr-namespace:BIF.Views"
         d:DesignHeight="120" d:DesignWidth="300">

    <ListBox ItemsSource="{Binding List}" 
        ScrollViewer.VerticalScrollBarVisibility="Hidden" 
        SelectedItem="{Binding CurrentPicture, Mode=TwoWay}" SelectionMode="Single">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal" />
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <ListBoxItem>
                    <view:Picture DataContext="{Binding}" />
                </ListBoxItem>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
</UserControl>

Views.Picture

<UserControl x:Class="BIF.Views.Picture"
         d:DesignHeight="200" d:DesignWidth="200">
    <Border BorderBrush="Black" BorderThickness="1" CornerRadius="4">
        <Image Source="{Binding FilePath}"/>
    </Border>
</UserControl>

I have already tried to set height of view:Picture manualy by adding:

Height="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type StackPanel}}, Path=ActualHeight}"

But thats not working 100% correct and I also do not want to set a fix height, because I want the PictureList to be resizeable.

Any suggestions are welcome.