VB.NET - Can't seem to set background stretch with radiobuttons

I've been trying some code for a while but I'm trying to code Radiobuttons that will change the Background Stretch when checked. I have been looking around with no luck :(

This is what I have so far, which doesn't work.

Dim imagebrush as new imagebrush()
    Private Sub BGNone_Checked(sender As Object, e As RoutedEventArgs) Handles BGNone.Checked
        If BGNone.IsChecked = True Then
            imagebrush.Stretch = Stretch.None
            Me.Background = imageBrush
        End If
    End Sub

    Private Sub BGFill_Checked(sender As Object, e As RoutedEventArgs) Handles BGFill.Checked
        If BGFill.IsChecked = True Then
            imagebrush.Stretch = Stretch.Fill
            Me.Background = imageBrush
        End If
    End Sub

Any help is appreciated :)