[求助]GDI+:为什么Picturebox1.Image还是Nothing?

folger

我定义了一个Graphics:
    Dim gph As Graphics

而且
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        gph = PictureBox1.CreateGraphics
    End Sub

但是就算我不停的
        gph.DrawLine(New Pen(color.Black), Int(Rnd() * 100), Int(Rnd() * 100), Int(Rnd() * 100), Int(Rnd() * 100))

Picturebox1.Image始终是Nothing。请问我怎样才能记录往Picturebox1上画上去的东西呢？谢谢！
有这方面的高手请指点