VBA to set logo in the header as Decorative

To also access the shapes in the Header(s) of the document, you would need to use something like:

Sub alttext()
    Dim shp As InlineShape
    Dim doc As Document
    Dim h As Long
    Dim s As Long
        
    Set doc = ActiveDocument
    
    For Each shp In doc.InlineShapes
        If shp.Width >= InchesToPoints(2) And shp.Height >= InchesToPoints(1) Then
            shp.AlternativeText = "Figure. Caption."
        Else
         shp.Decorative = True
        End If
    Next shp
    For s = 1 To doc.Sections.Count
        With doc.Sections(s)
            For h = 1 To .Headers.Count
                For Each shp In .Headers(h).InlineShapes
                    If shp.Width >= InchesToPoints(2) And shp.Height >= InchesToPoints(1) Then
                        shp.AlternativeText = "Figure. Caption."
                    Else
                        shp.Decorative = True
                    End If
                Next shp
            Next h
        End With
    Next s
End Sub