Image.Source binding problem

I'm having an issue with setting an Image.Source binding in the code behind file.

If create the elements in XAML and define the binding there, it works perfectly. But I need to be able to add the elements at runtime, and when I try to do this the Image doesn't update when the source is changed. Here's what I've got in the XAML:

<Canvas x:Name="parentCanvas"/>

And here's what have in the code-behind:

Canvas childCanvas;
WriteableBitmap source;

private void Method()
{
this.childCanvas = new Canvas();
Binding b = new Binding();
b.source = this.source;
Image img = new Image();
img.SetBinding(Image.SourceProperty, b);
this.childCanvas.Children.Add(img);
this.parentCanvas.Children.Add(this.childCanvas);
}

So again, when I bind the Image.Source in XAML to "img", it updates the Image anytime I change the WriteableBitmap. But adding them in code, it won't refresh. Also, if I asign a WriteableBitmap to "img", and THEN bind the new Image.Source to it, the Image shows up. But as soon as I assign a new WriteableBitmap to "img", the previous image stays displayed.