When you set the IsMdiContainer property of a form to true an MdiClient control is added to your form. This is the grey area that you can see and it is what hosts the child forms, not the parent form itself. You are not provided an explicit reference to this MdiClient control because you are not supposed to touch it. If you place a PictureBox on your form it doesn't "leak" through anything. It is physically sitting on top of the child forms in the z-order. If you call SendToBack on the PictureBox then it will drop to the bottom of the z-order and will be hidden behind the MdiClient. If you want to provide a background for the client area of the parent form then you need to get a reference to the MdiClient control, which can only be done at run time, e.g.
VB Code:
  1. Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

  2.     For Each ctl As Control In Me.Controls

  3.         If TypeOf ctl Is MdiClient Then

  4.             ctl.BackColor = Color.Red

  5.             Exit For

  6.         End If

  7.     Next

  8. End Sub