in effetti è un po' "laborioso"... ma sono riusito a trovarlo, il metodo

codice:
Private Sub MDIForm_Resize()

Static tmpPic As PictureBox
Static bkpImg As StdPicture
Static Exists As Boolean

On Error Resume Next

If Not Exists Then
    Set tmpPic = Me.Controls.Add("VB.PictureBox", "tmp")
    Set bkpImg = Me.Picture
    Exists = True
End If

With tmpPic
    .Cls
    .BorderStyle = 0
    .AutoRedraw = True
    .Move 0, 0, Me.ScaleWidth, Me.ScaleHeight
    .PaintPicture bkpImg, 0, 0, Me.ScaleWidth, Me.ScaleHeight
    Me.Picture = .Image
End With

End Sub
Boolean