Puedes hacerlo así:
Código:
' By MaxExtreme (Tamaño de Imagen)
Private Const IMAGESIZE = 0.566893424036281
Private Sub MDIForm_Resize()
On Error Resume Next
Dim ImageWidth As Single
Dim ImageHeight As Single
picStretch.Visible = False
picStretch.AutoRedraw = True
ImageWidth = picStretch.Picture.Width * IMAGESIZE
ImageHeight = picStretch.Picture.Height * IMAGESIZE
picStretch.PaintPicture picStretch.Picture, 0, 0, Me.Width, Me.Height, 0, 0, ImageWidth, ImageHeight
Set Me.Picture = picStretch.Image
End Sub
Saludos
P.S.: Debes tener un PictureBox llamado picStretch, en ella debes colocar la imagen que quieres colocar como fondo de la Ventana...