...
<ListBox.ItemTemplate>
  <DataTemplate>
    <Image Source="{Binding ImageUrl}" Height="90" Width="90" Stretch="UniformToFill" />
  </DataTemplate>
</ListBox.ItemTemplate>
...

public static readonly DependencyProperty DecodingSourceProperty = DependencyProperty.RegisterAttached(
DecodingSourcePropertyName,
typeof (Uri),
typeof (Image),
new PropertyMetadata(null, OnDecodingSourcePropertyChanged));

static void OnDecodingSourcePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
    var img = d as Image;
    double height = img.Height;
    double width = img.Width;
    var uri = (Uri)e.NewValue;
    var bmp = new WriteableBitmap((int)width, (int)height);

    ThreadPool.QueueUserWorkItem(callback => {
        var web = new WebClient();
        web.OpenReadCompleted += (sender, evt) => {
            bmp.LoadJpeg(evt.Result);
            evt.Result.Dispose();
            Deployment.Current.Dispatcher.
            BeginInvoke(() = > {
                img.Source = bmp;
            });
        };
        web.OpenReadAsync(uri);
    }
    });
}

<Image helpers:ImageExt.DecodingSource="{Binding ImageUrl}" Height="90" Width="90" Stretch="UniformToFill" />