Picturebox1に円を描いた後、Button1で円を追加したいと思います。
以下のコードだと前の円が消えてしまうのですが、どうすればいいですか？
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim canvas As New Bitmap(PictureBox1.Width, PictureBox1.Height)
Dim g As Graphics = Graphics.FromImage(canvas)
g.DrawArc(Pens.Black, 20, 20, 20, 20, 0, 360)
PictureBox1.Image = canvas
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim canvas As New Bitmap(PictureBox1.Width, PictureBox1.Height)
Dim g As Graphics = Graphics.FromImage(canvas)
g.DrawArc(Pens.Black, 20, 20, 10, 10, 0, 360)
PictureBox1.Image = canvas
End Sub