Pre-load images onto DataTemplate

Hello, everyone:

Another question about binding. Lately, I could figure it out how to put an image in a normal page while I wait an answer from the server to know if an image exist or not:

• I made a string on my ViewModel for Binding on my page:
  string _imageProductUrl;

  public string ImageProductUrl
  {
      get { return _imageProductUrl; }
      set
      {
          _imageProductUrl = value;
          OnPropertyChanged("ImageProductUrl");
      }
  }
  

• In the ViewModel constructor I setted ImageProductUrl with a local file representing "no image loaded"

• Then I created this command. It checks if a file exists in the server and, if we get a 200 response, it change the ImageProductUrl value:
  Command _CheckNetFile;

  public Command CheckNetFile
  {
      get { return _CheckNetFile ?? (_CheckNetFile = new Command(ExecuteCheckNetFile)); }
  }
  
  async void ExecuteCheckNetFile ()
  {
      string _fileUrl = myUrlDefinedElsewhere;
      //Request creation
      HttpClient httpClient = new HttpClient();
      HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Head, new Uri(_fileUrl));
      //Request send
      HttpResponseMessage response = await httpClient.SendAsync(request);
      if (response.StatusCode == HttpStatusCode.OK)
          ImageProductUrl = _fileUrl;
  }
  

• And finally, in the code behind my xaml page, I overwrote the OnAppearing method like this:
  protected override void OnAppearing()
  {
  base.OnAppearing();
  ViewModel.CheckNetFile.Execute(ViewModel.CatalogProduct);
  }

I don't know if this code is stylish or not, but it works. But this is not the subject of my question.
Now I have another page: the list of products. This page uses a ListView, but this list is using a DataTemplate written on a View file.

    <DataTemplate>
        <ViewCell StyleId="disclosure">
            <ContentView Padding="10,10">
                <StackLayout Orientation="Horizontal" Spacing="0">
                    <AbsoluteLayout>
                        <ActivityIndicator
                        BindingContext="{x:Reference ThumbnailImage}"
                        IsEnabled="{Binding IsLoading}"
                        IsVisible="{Binding IsLoading}"
                        IsRunning="{Binding IsLoading}"/>
                    <Image
                        x:Name="ThumbnailImage"
                        Source="{Binding ThumbnailImageUrl, Converter={StaticResource itemImageToUrlConverter}}" />
                    </AbsoluteLayout>
                    <StackLayout Spacing="0">
                        <Label
                        Text="{Binding Name}" />
                        <Label
                        Text="{Binding Description}"/>
                    </StackLayout>
                </StackLayout>
            </ContentView>
        </ViewCell>
    </DataTemplate>

As you can see, what I'm doing now is using a converter to get the name of the file and convert it to a valid URL, but I would like to do something similar as I did on the detail page before, to show a default image if the file is not existing on the server, but because it's using a DataTemplate, I can't write a command, or modify the source of "ThumbnailImage" Image, or even use an async method on converter. So if someone can say me how to do it, I'll appreciate it.

PD: I hope you have enough data to work, because I don't know how much code can I share with you ^^U