Sub TestCode()
 
'Open Existing PowerPoint
Set oPAe = CreateObject("PowerPoint.Application")
With oPAe
    .Visible = True
    Set oPPe = .Presentations.Open("C:\Users\User\Documents\Test\Test.pptx")
End With
 
'Insert two pictures (2nd iteration fails)
MyPicture = "C:\Users\User\Documents\Test\MyPic1.jpg": GoSub InsertPicture 'MyPic1 is inserted, resized and centered (this iteration is ok)
MyPicture = "C:\Users\User\Documents\Test\MyPic2.jpg": GoSub InsertPicture 'MyPic2 inserts but resize and centering is applied to MyPic1 (this iteration fails)
Exit Sub
 
InsertPicture:
With oPAe
    .ActiveWindow.View.GotoSlide (1) 'this line makes testing easier otherwise not required
    Set oPSe = oPPe.Slides(1)
   'Add Picture
    Set oShapee = oPSe.Shapes.AddPicture(MyPicture, msoFalse, msoTrue, 1, 1, -1, -1) '(Filename, LinkToFile, SaveWithDocument, Left, Top, Width, Height)
 
   'Do some work (code works below here but operates on MyPic1 both iterations)
   'Size Picture
    Set oPicturee = oPSe.Shapes(oPSe.Shapes.Count)
    oPicturee.LockAspectRatio = True
    oPicturee.Width = 500 'value is arbitrary
   'Center picture
    With oPAe.ActivePresentation.PageSetup
        oPicturee.Left = (.SlideWidth \ 2) - (oPicturee.Width \ 2)
        oPicturee.Top = (.SlideHeight \ 2) - (oPicturee.Height \ 2)
    End With
End With
Return
 
End Sub

Sub TestCode()
Dim objPPTApp As Object
Dim objPPPres As Object
Dim objPPSld As Object
'Open Existing PowerPoint
Set objPPTApp = CreateObject("PowerPoint.Application")
With objPPTApp
    .Visible = True
    Set objPPPres = .Presentations.Open("C:\Users\John\Documents\Test\Test.pptx")
End With
Set objPPSld = objPPPres.Slides(1)
Call addpic(objPPPres, objPPSld, "C:\Users\John\Documents\test\Pic1.jpg")
Call addpic(objPPPres, objPPSld, "C:\Users\John\Documents\test\Pic2.jpg")
End Sub

Sub addpic(opres As Presentation, osld As Slide, strPath As String)
Dim opic As Shape
Set opic = osld.Shapes.AddPicture(strPath, False, True, 1, 1, -1, -1)
With opic
.LockAspectRatio = True
.Width = 500
With opres.PageSetup
        opic.Left = (.SlideWidth \ 2) - (opic.Width \ 2)
        opic.Top = (.SlideHeight \ 2) - (opic.Height \ 2)
End With
End With
End Sub