How do I resize images using vba code (powerpoint 2010) so they are center scaled?

I've written vba code to reduce the width of all images in a deck by 25%. But they default to reducing these images by the right side. How can I incorporate into my code that the images scale back 25% from their center and not the right side. I know there is a vba term ScaleFromMiddle but don't know how to incorporate it into my code.
Thank you.

Sub Scale()
Dim Sl As Slide
Dim Sh As Shape
For Each Sl In ActivePresentation.Slides
    For Each Sh In Sl.Shapes
        Sh.LockAspectRatio = False
    Next
Next
Dim oshp As Shape
Dim osld As Slide
Dim dblscaleamount As Double

dblscaleamount = 0.75
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.Type = msoPicture Or _
   oshp.Type = msoChart Or _
   oshp.Type = msoTable Or _
   oshp.Type = msoAutoShape Or _
   oshp.Type = msoGroup Then
oshp.width = oshp.width * dblscaleamount
oshp.Height = oshp.Height
End If
Next oshp
Next osld
End Sub