Try
Code:
Dim WM As Shape
With Selection.Sections(1).Headers(wdHeaderFooterPrimary)
If .Shapes.count > 0 Then
.Shapes(1).Delete
End If
Set WM = .Shapes.AddPicture(filename:="E:\IT\MS Office 2013\PREP Watermark.png")
With WM
With .PictureFormat
.Brightness = 0.5
.Contrast = 0.5
End With
.LockAspectRatio = True
.Height = CentimetersToPoints(14.34)
.Width = CentimetersToPoints(15.92)
.WrapFormat.Type = 5
.RelativeHorizontalPosition = wdRelativeVerticalPositionMargin
.RelativeVerticalPosition = wdRelativeVerticalPositionMargin
.Left = wdShapeCenter
.Top = wdShapeCenter
End With
End With