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