Public Class Form1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        PictureBox1.Dock = DockStyle.Fill
        PictureBox1.Image = New Bitmap(PictureBox1.Width, PictureBox1.Height)
    End Sub
 
    Private Sub PictureBox1_MouseDown1(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
        Dim rnd As New Random(Environment.TickCount)
        Dim rec As New Rectangle(e.X, e.Y, 40, 40)
        Using g As Graphics = Graphics.FromImage(PictureBox1.Image), _
              sb As New SolidBrush(Color.FromArgb(rnd.Next(256), rnd.Next(256), _
              rnd.Next(256)))
            g.FillRectangle(sb, rec)
            g.DrawRectangle(Pens.Black, rec)
        End Using
        PictureBox1.Invalidate()
    End Sub
End Class