Adding an image to multiple gridview headers

I'm attempting to add an image into the headers of specific columns in my gridview.  The code below doesn't throw an error but only adds the GIF to the final column referenced in the sub.  Can anyone offer any advice as to why that might happen??

Sub addImages(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
      If e.Row.RowType = DataControlRowType.Header Then
            Dim sortImage As New Image()
            sortImage.ImageUrl = "~/images/sorticon.gif"
            e.Row.Cells(1).Controls.Add(sortImage) ' <-- not added
            e.Row.Cells(2).Controls.Add(sortImage) ' <-- not added
            e.Row.Cells(3).Controls.Add(sortImage) ' <-- not added
            e.Row.Cells(4).Controls.Add(sortImage) ' <-- added!
      End If
End Sub