How can I able to see the image in my picturebox? - vb.net-2010

Question

I have used the following code to draw what I want but when I used this code I cannot see the image I set on my picturebox. What should I do to draw on the picture box with an image? Please help me with this.
Public Class Form1
Dim draw As Boolean
Dim DrawColor As Color = Color.Black
Dim DrawSize As Integer = 6
Dim bmp As Bitmap
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
cbxSize.SelectedIndex = 2
bmp = New Bitmap(pbDraw.Width, pbDraw.Height)
pbDraw.Image = bmp
Dim down = False
End Sub
Private Sub PaintBrush(X As Integer, Y As Integer)
Using g As Graphics = Graphics.FromImage(pbDraw.Image)
g.FillRectangle(New SolidBrush(DrawColor), New Rectangle(X, Y, DrawSize, DrawSize))
End Using
pbDraw.Refresh()
End Sub
Drawing event
Private Sub pbtest_MouseDown(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles pbDraw.MouseDown
draw = True
PaintBrush(e.X, e.Y)
End Sub
Private Sub pbtest_MouseMove(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles pbDraw.MouseMove
If draw = True Then
PaintBrush(e.X, e.Y)
End If
End Sub
Private Sub pbtest_MouseUp(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles pbDraw.MouseUp
draw = False
End Sub
Please help me