Wpf ImageViewer

witam,zacząłem robić przeglądarkę zdjęć w wpf-ie i stanąłem pewnym momencie.nie działa mi metoda która wyświetli zaznaczone zdjęcie z listy miniatur w panelu głównym,wypluwa komunikat:

Object reference not set to an instance of an object.
Co zrobiłem nie tak?

 
<Window x:Class="przeg_test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

    <Window.Resources>
        <DataTemplate x:Key="ImageDataTemplate">
            <Image Source="{Binding zdjecie}" Width="125"
                   Height="125" />
        </DataTemplate>
    </Window.Resources>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <ListBox Name="lista"  Grid.Row="1"  HorizontalAlignment="Stretch" Margin="5" VerticalAlignment="Stretch"
                 ItemsSource="{Binding}" ItemTemplate="{StaticResource ImageDataTemplate}" SelectionChanged="lista_SelectionChanged">
        </ListBox>
        <Button Click="Dodaj_zdjecie" Content="Dodaj zdjęcie" Height="23" Width="76" Grid.ColumnSpan="2"></Button>
        <Grid Grid.Column="1" Grid.Row="1" HorizontalAlignment="Stretch" Margin="5" Name="grid1" VerticalAlignment="Stretch">
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
                <RowDefinition Height="auto" />
            </Grid.RowDefinitions>
            <!-- PANEL GLOWNY DO WYSWIETLANIA ZDJEC -->
            <Image  Grid.Row="0" HorizontalAlignment="Center" Margin="5" Name="image" Stretch="UniformToFill" VerticalAlignment="Center"/>
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Name="stackPanel1" Grid.Row="1">
                <Button  Content="Odwróć" Height="23" Name="odwroc" Width="76" />
                <Button  Content="Usuń" Height="23" Name="usun" Width="76" />
            </StackPanel>
        </Grid>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO;
using System.Windows.Media.Effects;
using System.Collections.ObjectModel;

namespace przeg_test
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        ObservableCollection<Zdjecia> a = new ObservableCollection<Zdjecia>
        {

        };

        public MainWindow()
        {
            InitializeComponent();
            lista.ItemsSource = a;
        }

        string wybrany_plik;

        private void Dodaj_zdjecie(object sender, RoutedEventArgs e)
        {
            System.Windows.Forms.OpenFileDialog plik = new System.Windows.Forms.OpenFileDialog();
            plik.InitialDirectory = "c:\\";
            plik.Filter = "Image files (*.jpg)|*.jpg|All Files (*.*)|*.*";
            plik.RestoreDirectory = true;

            if (plik.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                wybrany_plik = plik.FileName;
                a.Add(new Zdjecia { zdjecie = new BitmapImage(new Uri(wybrany_plik)) });
            }
        }

        //wyswietla zaznaczone zdjecie z listy miniatur w panelu glownym
        private void lista_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            BitmapImage img = (sender as ListBox).SelectedItem as BitmapImage;
            image.Source = new BitmapImage(new Uri(img.ToString()));
        }
       
    }

    class Zdjecia
    {
        public BitmapImage zdjecie
        {
            get;
            set;
        }
    }
}