Как сделать постепенное затемнение картинки в PictureBox

Public Class frmПроба
Dim holst As Bitmap
Private Sub frmПроба_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
holst = New Bitmap("E:\ПроектыVB10\Проба\ВО!.JPG")
pic.Image = holst
End Sub
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Call LightOrDark(0.9)
End Sub
Private Sub LightOrDark(ByVal f As Single)
Dim r, g, b, x, y As Int16, clr As Color
Dim graph As Graphics = Graphics.FromImage(holst)
Dim grf As Graphics = pic.CreateGraphics
For y = 0 To pic.ClientSize.Height - 1
For x = 0 To pic.ClientSize.Width - 1
' Получить цвет
clr = holst.GetPixel(x, y)
r = clr.R Mod 256 : g = (clr.G \ 256) Mod 256 : b = clr.B \ 256 \ 256
'Уменьшить/увеличить яркость
r = r * f : g = g * f : b = b * f
If r < 0 Then r = 0
If g < 0 Then g = 0
If b < 0 Then b = 0
clr = Color.FromArgb(r, g, b)
holst.SetPixel(x, y, clr)
Next x
Next y
grf.DrawImage(holst, 0, 0) : pct.Image = holst
End Sub
End Class

 Dim a As Integer = 0
 
    Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick
        If a < 255 Then
            PictureBox1.CreateGraphics.FillRectangle(New SolidBrush(Color.FromArgb(a, 0, 0, 0)), 0, 0, PictureBox1.Width, PictureBox1.Height)
            a += 1
        End If
    End Sub