Items in listbox disappears after click

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