It's not going to show a random number if you just iterate through 1 to 7...
I haven't used .NET in years, but try this:

Code:
Dim rndVal As New Random(System.DateTime.Now.Millisecond)
        Dim i As Integer
        Dim BGimage(6)
        BGimage(0) = Resources.image_1
        BGimage(1) = Resources.image_2
        BGimage(2) = Resources.image_3
        BGimage(3) = Resources.image_4
        BGimage(4) = Resources.image_5
        BGimage(5) = Resources.image_6
        BGimage(6) = Resources.image_7
            Me.BackgroundImage = BGimage(rndVal.Next(0,6))