Items in listbox disappears after click

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C#

WPF

Hi,
I've a listbox that shows some pictures.
this is the xaml code

<ListBox Height="500" Name="lbThumbnails" IsEnabled="False" SelectionMode="Single" ItemsSource="{Binding Source}" >
                           <ListBox.ItemsPanel>
                               <ItemsPanelTemplate>
                                   <StackPanel></StackPanel>
                               </ItemsPanelTemplate>
                           </ListBox.ItemsPanel>
                           <ListBox.ItemTemplate>

                               <DataTemplate>
                                   <Border BorderBrush="#FFB4B4B4" Opacity="0.7" BorderThickness="2" CornerRadius="5" >
                                       <Image Source="{Binding UriSource}"  Stretch="Fill" Width="100" Height="100" Margin="20"  />
                                   </Border>
                               </DataTemplate>
                           </ListBox.ItemTemplate>
                       </ListBox>

this is the code-behind

pictures = new List<BitmapImage>();
           try
           {
               DirectoryInfo pictureImageDir = new DirectoryInfo("Thumbnails");
               foreach (FileInfo koiImageFile in pictureImageDir.GetFiles("*.jpg"))
               {
                   Uri uri = new Uri(koiImageFile.FullName);
                   pictures.Add(new BitmapImage(uri));
               }
               lbThumbnails.ItemsSource = pictures;
           }
           catch (Exception ex)
           {
               System.Windows.MessageBox.Show("Errore non previsto");
           }

The loading works but after the user click on an item, all the items disappears.
This is the first time that I see this kind of error and i've no idea of what is the problem.
PS: I've tried to manage the selection changed event of listbox, but the execution flow doesn't enter in that code. It's very strange.
Can you help me?

Greats