Option Explicit

Sub TestCode()
'Declare
Dim oPA As PowerPoint.Application
Dim oPP As PowerPoint.Presentation
Dim oPS As PowerPoint.Slide
Dim oShape As PowerPoint.Shape
Dim oPicture As PowerPoint.Shape
Dim ExistingPPT As String
Dim MyPicture As String
'Set path and fielname of existing PowerPoint
ExistingPPT = "C:\Users\John\Desktop\Test.pptx" 'make sure presentation has one blank slide.
 
'Set path and filename of picture to be imported into ppt
MyPicture = "C:\Users\John\Desktop\Test.jpg"
 
'Open Existing PowerPoint
Set oPA = CreateObject("PowerPoint.Application")
With oPA
    'Open existing PowerPoint (THIS BLOCK WORKS)
    .Visible = True
Set oPP = .Presentations.Open(ExistingPPT)
    .ActiveWindow.View.GotoSlide (1) 'this line makes testing easier otherwise not required
Set oPS = oPP.Slides(1)
   'Add Picture (e.g. import it into PPT) (THIS BLOCK DOES NOT WORK)
    Set oShape = oPS.Shapes.AddPicture(MyPicture, msoFalse, msoTrue, 1, 1, -1, -1) '(Filename, LinkToFile, SaveWithDocument, Left, Top, Width, Height)
 
   'Size Picture
    Set oPicture = oPS.Shapes(oPS.Shapes.Count)
'The next two line are not needed using -1 (True) when adding already did this
   ' oPicture.ScaleHeight 1, msoTrue
   ' oPicture.ScaleWidth 1, msoTrue
    oPicture.LockAspectRatio = True
    oPicture.Width = 750 '750 is arbitrary
 
   'Center picture
    With oPA.ActivePresentation.PageSetup
        oPicture.Left = (.SlideWidth \ 2) - (oPicture.Width \ 2)
        oPicture.Top = (.SlideHeight \ 2) - (oPicture.Height \ 2)
    End With
End With
 
End Sub