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

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

Code:

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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

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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:
Attachment 85350

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

I hunted the web for anything to do with forcing a WPF to render, including this promising looking Refresh method extension for UIElements; but nothing worked. Then I tried adding this at the end of the sub:

Code:

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Me.Left+=1
Me.Left-=1

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and it worked: the largest images I could find rendered correctly. (Of course I could also add and subtract 1 alternately in each call of the sub.) In fact, anything exceeding Me.Left+=0.4 or less than Me.Left-=0.6 seemed to work. And that suggests to me that it has something to do with rendering on a hardware pixel boundary.

My question stays open however. Can anyone suggest a "correct" way to deal with this problem? I should add I'm using WinXPh with 3GB memory; maybe the problem won't reproduce on other systems.

BB