How to force UI display to update in Prism, C#?

Question

I want to develop a video player with Prism+WPF+C#.
However ui is not updated, when the image data is update.

In Xaml

 <Image x:Name="VideoDisplay" Source="{Binding ImageData}"/>

In ViewModel

...
        void update(BitmapSource src) 
        {
           this.DisplayImageData = src;
        }

        public BitmapSource ImageData
        {
            get { return this.DisplayImageData; }
        }
...

The image is reflected when constructor is called. After constructer, I want to render the image. How do I force ui display to update?