Sub ReplaceImage()
Dim oHeader As HeaderFooter
Dim oRng As Range
    For Each oHeader In ActiveDocument.Sections(1).Headers
        If oHeader.Exists Then
            If oHeader.Range.InlineShapes.Count > 0 Then
                Set oRng = oHeader.Range.InlineShapes(1).Range
                oRng.Text = ""
                oRng.InlineShapes.AddPicture _
                        FileName:="C:\Path\filename.jpg", _
                        LinkToFile:=False, _
                        SaveWithDocument:=True
            End If
        End If
    Next oHeader
lbl_Exit:
    Set oHeader = Nothing
    Set oRng = Nothing
    Exit Sub
End Sub