How do I clean up pending weak events?

I am using main window as context, which has an image bound to a byte[], like this

MainWindow : INotifyChanged

{

    byte[] _image;
    ImageContent
     {
         get {return _image;}
     }

    Main()

    {
         this.Context = this;

    }

    private void Polling()
    {
        while()
        {
        //download image
        NotifyPropertyChanged("ImageContent");
        }       
    }

    NotifyPropertyChanged(string propertyname)
    {
       if(PropertyChanged != null)
       {
           PropertyChanged(propertyname);
       }
    }
}

and the xaml has
<image source = {binding = ImageContent} />

and the problem is, I am downloading 10 images in 1 second, if I change the url to download another image, ImageControl will flicker between different image from the two sources for a while.

I think this is because my polling is faster than Image control update, and some event handler is queued inside WeakEventManager, I have search and be told that there is no easy way to clean up queued event.

I tried WeakEventManager.GetCurrentManager(), Visual Studio tells me that there is no such method. I have tried OnLoad, OnSourceChaneged, OnTargetChanged, non of them was fired weak event.

Could anyone please tell me how do I resolve this? with or without weak event. ?

I would like to get rid of that flickering.