[SaswatPadhi]

This should work in vb.net :

Private Sub btnGo_Click(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles btnGo.Click
    Dim bm As New Bitmap(picCanvas.Image)
    Dim X As Integer
    Dim Y As Integer
    Dim clr As Integer

    For X = 0 To bm.Width - 1
        For Y = 0 To bm.Height - 1
            clr = (CInt(bm.GetPixel(X, Y).R) + _
                   bm.GetPixel(X, Y).G + _
                   bm.GetPixel(X, Y).B) \ 3
            bm.SetPixel(X, Y, Color.FromArgb(clr, clr, clr))
        Next Y
    Next X

    picCanvas.Image = bm
End Sub