Try something based on:
Sub UpdateHeaderLogos()
Application.ScreenUpdating = False
Dim Rng As Range, Sctn As Section, HdFt As HeaderFooter, StrNm As String
With Application.Dialogs(wdDialogInsertPicture)
If .Show = True Then StrNm = .Name
End With
If StrNm = "" Then Exit Sub
With ActiveDocument
For Each Sctn In .Sections
For Each HdFt In Sctn.Headers
With HdFt
If .Exists = True Then
If (Sctn.Index = 1) Or (.LinkToPrevious = False) Then
If .Range.Tables.Count = 1 Then
With .Range.Tables(1).Range.Cells(1).Range
If .InlineShapes.Count = 1 Then
Set Rng = .InlineShapes(1).Range
.InlineShapes(1).Delete
.InlineShapes.AddPicture FileName:=StrNm, Range:=Rng
End If
End With
End If
End If
End If
End With
Next
Next
End With
Application.ScreenUpdating = True
End Sub