Detecting Item visibility in a VirtualizingStackPanel for lazy loading images - windows-store-apps

Question

Here is my scenario:
I have a Gridview that shows data retrieved from a network service. The number of data may be quite large. So, the VirtualizingStackPanel is used to display the content. Some of those data may require retrival of images when they are rendered(lazy loading)
My (very limited) understanding is that the VirtualStackPanel will automatically request data as the grid requiring that data is being rendered. If the element that needs to be rendered is of image type, I send a placeholder image and submit an async network call to retrieve that image.
But, I notice that the calls to render all the element is being received and consequently a deluge of network calls to retrieve images are sent (esp if most of the items are of image type).
Question: Is there something I should be configuring in code (XAML or C#) for the VirtualizingStackPanel or should go the route of detecting if the item is really being displayed and then issue network call?
Here is my XAML.
<GridView
x:Name="itemGridView"
AutomationProperties.AutomationId="ItemGridView"
AutomationProperties.Name="Grouped Items"
Grid.RowSpan="3"
Padding="116,137,40,46"
ItemsSource="{Binding Source={StaticResource groupedItemsViewSource}}"
VirtualizingStackPanel.VirtualizationMode="Standard"
ItemTemplateSelector="{StaticResource CellStyleSelector}"
ItemClick="ItemView_ItemClick"
IsItemClickEnabled="True"
SelectionMode="None"
IsSwipeEnabled="false">
<GridView.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
</GridView.ItemsPanel>
Here is the Item's Property which is bound to the Image in XAML
private ImageSource _image = null;
private String _imagePath = null;
public ImageSource ThumbImage
{
get
{
// Check if the image is already cached
if (this._image == null)
{
// Setup a placehoder image
this._imagePath = "Assets/metro_photo_55#2x.png";
this._image = new BitmapImage(new Uri(Defs.BaseUri, this._imagePath));
// Async call to download thumb image from server
this.getThumb();
}
return this._image;
}
private set
{
this.SetProperty(ref this._image, value);
}
}
Here is the async call to download the thumb
private async void getThumb()
{
// Get image and store
Network network = new Network();
// Since this is a bound property. this action updates the UI
this.ThumbImage = await network.getImage(this._rawData[Defs.PATH_KEY], true);
Debug.WriteLine(this._rawData[Defs.PATH_KEY] + " - Thumb retrieved!");
}