Try it this way instead:
Sub TestCenterImage()
Dim osld As Slide
Dim oShp As Shape
For Each osld In ActivePresentation.Slides
'If osld.SlideIndex > 1 Then Exit Sub 'I don't know if I need this line
For Each oShp In osld.Shapes
If CheckIsPic(oShp) = True Then 'Making sure that we're only working with images
CenterOnSlide oShp
'End With
End If
Next oShp
Next osld
End Sub
Function CheckIsPic(oShp As Shape) As Boolean
If oShp.Type = msoPicture Then CheckIsPic = True
If oShp.Type = msoPlaceholder Then
If oShp.PlaceholderFormat.ContainedType = msoPicture Then CheckIsPic = True
End If
End Function
Sub CenterOnSlide(oShp As Shape)
Dim sngSlideWidth As Single
Dim sngSlideHeight As Single
sngSlideWidth = ActivePresentation.PageSetup.SlideWidth
sngSlideHeight = ActivePresentation.PageSetup.SlideHeight
oShp.Left = sngSlideWidth / 2 - oShp.Width / 2
oShp.Top = sngSlideHeight / 2 - oShp.Height / 2
End Sub