Problems with GridView

<Page
    x:Name="pageRoot"
    x:Class="MicrosoftFWLinks.Favorites"
    DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MicrosoftFWLinks"
    xmlns:common="using:MicrosoftFWLinks.Common"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" Loaded="pageRoot_Loaded">

    <Page.Resources>
        <!-- TODO: Delete this line if the key AppName is declared in App.xaml -->
        <CollectionViewSource x:Name="cvs"
                              Source="{Binding FavoritesColl}"
                              IsSourceGrouped="False"
                              />
        <DataTemplate x:Name="TElement">
            <Grid Width="500" Height="309" >
                <Border BorderThickness="2" BorderBrush="Black">
                    <Image Source="{Binding Image}" Stretch="Fill" />
                </Border>
                <TextBlock Text="{Binding Title}" VerticalAlignment="Bottom"/>
            </Grid>
        </DataTemplate>
    </Page.Resources>

    <!--
        This grid acts as a root panel for the page that defines two rows:
        * Row 0 contains the back button and page title
        * Row 1 contains the rest of the page layout
    -->
    <Grid removed="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid.ChildrenTransitions>
            <TransitionCollection>
                <EntranceThemeTransition/>
            </TransitionCollection>
        </Grid.ChildrenTransitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="140"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!-- Back button and page title -->
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="120"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Button x:Name="backButton" Margin="39,59,39,0" Command="{Binding NavigationHelper.GoBackCommand, ElementName=pageRoot}"
                        Style="{StaticResource NavigationBackButtonNormalStyle}"
                        VerticalAlignment="Top"
                        AutomationProperties.Name="Back"
                        AutomationProperties.AutomationId="BackButton"
                        AutomationProperties.ItemType="Navigation Button"/>
            <TextBlock x:Name="pageTitle" Style="{StaticResource HeaderTextBlockStyle}" Grid.Column="1" 
                        IsHitTestVisible="false" TextWrapping="NoWrap" VerticalAlignment="Bottom" Margin="0,0,30,40">
                <Run FontFamily="Segoe UI Symbol" Text="" />
                <Run Text="  Favorites" />
            </TextBlock>
        </Grid>
        <GridView Grid.Row="1" ItemTemplate="{Binding TElement}" ItemsSource="{Binding Source={StaticResource cvs}}>

        </GridView>
    </Grid>
</Page>

namespace MicrosoftFWLinks
{
    /// <summary>
    /// Pagina base che fornisce caratteristiche comuni alla maggior parte delle applicazioni.
    /// </summary>
    public sealed partial class Favorites : Page
    {

        private NavigationHelper navigationHelper;
        private ObservableDictionary defaultViewModel = new ObservableDictionary();

               public ObservableDictionary DefaultViewModel
        {
            get { return this.defaultViewModel; }
        }

                public NavigationHelper NavigationHelper
        {
            get { return this.navigationHelper; }
        }


        public Favorites()
        {
            this.InitializeComponent();
            this.navigationHelper = new NavigationHelper(this);
            this.navigationHelper.LoadState += navigationHelper_LoadState;
            this.navigationHelper.SaveState += navigationHelper_SaveState;
        }

                private void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
        }

               private void navigationHelper_SaveState(object sender, SaveStateEventArgs e)
        {
        }

        
        private async void pageRoot_Loaded(object sender, RoutedEventArgs e)
        {
            IReadOnlyList<StorageFile> c = await KnownFolders.PicturesLibrary.GetFilesAsync();
            List<StorageFile> images = new List<StorageFile>();
            foreach (StorageFile s in c)
            {
                if (s.FileType == ".png")
                    images.Add(s);
            }
            foreach (StorageFile s in images)
            {
                FavoritesColl.Add(new Favorite(new Uri(s.Path, UriKind.Absolute), s.DisplayName));
            }
        }
        ObservableCollection<Favorite> FavoritesColl = new ObservableCollection<Favorite>();
        public class Favorite
        {
            public Uri Source { get; set; }
            public string Title { get; set; }
            public Favorite(Uri source, string text)
            {
                Title = text;
                Source = source;
            }
        }
    }
}