When dealing with this sort of thing, it's easiest to record a macro, take a look at the Word-generated code, then adapt for your particular need.

The recorded macro:

Code:
  ActiveDocument.Sections(1).Range.Select
  ActiveWindow.ActivePane.View.SeekView = wdSeekCurrentPageHeader
  Selection.HeaderFooter.Shapes.AddPicture(FileName:="C:\MyPicture.png", _
    LinkToFile:=False, SaveWithDocument:=True).Select
  Selection.ShapeRange.Name = "WordPictureWatermark26334388"
  Selection.ShapeRange.PictureFormat.Brightness = 0.85
  Selection.ShapeRange.PictureFormat.Contrast = 0.15
  Selection.ShapeRange.LockAspectRatio = True
  Selection.ShapeRange.Height = InchesToPoints(2.79)
  Selection.ShapeRange.Width = InchesToPoints(6.5)
  Selection.ShapeRange.WrapFormat.AllowOverlap = True
  Selection.ShapeRange.WrapFormat.Side = wdWrapNone
  Selection.ShapeRange.WrapFormat.Type = 3
  Selection.ShapeRange.RelativeHorizontalPosition = _
    wdRelativeVerticalPositionMargin
  Selection.ShapeRange.RelativeVerticalPosition = _
    wdRelativeVerticalPositionMargin
  Selection.ShapeRange.Left = wdShapeCenter
  Selection.ShapeRange.Top = wdShapeCenter
  ActiveWindow.ActivePane.View.SeekView = wdSeekMainDocument