[WPF]Window temporarily corrupted after OpenFileDialog to load large image

[boops boops]

In the following code, I use an OpenFileDialog to select an image to place on Canvas1, which I added in the designer.

Class MainWindow

Private _Image As New Image

Private Sub Button1_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Button1.Click
	Dim ofd As New Microsoft.Win32.OpenFileDialog
	If ofd.ShowDialog = True Then
		Canvas1.Children.Remove(_Image)
		Dim bi As New BitmapImage
		bi.BeginInit()
		bi.UriSource = New Uri(ofd.FileName)
		bi.EndInit()	
		_Image.Source = bi
		Canvas1.Children.Add(_Image)
		'Me.InvalidateArrange()  'tried this, didn't help.
	End If
End Sub

End Class

This works fine except for very large images -- usually somewhere over 32 MegaPixels, though that varies from run to run. When it goes wrong, the Window fails to render. Here's an example:


If I drag other windows over the corrupt window, they leave trails. But immediately I start to drag the WPF window itself by its title bar, it renders correctly. Can anyone suggest a way to deal with this in code?

BB