Calculate height and width of GIF/JPG files

Calculate height and width of GIF/JPG files(2)    f = Freefile()
   On Error Resume Next
   Open fileName For Input As #f
   On Error Goto 0
   If Err <> 0 Then
      ImageSize = retVal  ' File name incorrect - return zero for both the height and width
      Exit Function
   End If
   If Lcase(Right(fileName, 3)) = "gif" Then
      ' GIF's height and width stored in a fixed location
      header = Input(10, f)
      wHi = Mid(header, 8, 1)
      wLo = Mid(header, 7, 1)
      hHi = Mid(header, 10, 1)
      hLo = Mid(header, 9, 1)
      w = Asc(wHi) * 256 + Asc(wLo)
      h = Asc(hHi) * 256 + Asc(hLo)
   Elseif Lcase(Right(fileName, 3)) = "jpg" Then
      ' JPG's stored in a variable location. The code has been verified with JFIF
      ' file format (the most common format)