须创建bitmap,关联到pictureBox1.image上。

再在pictureBox1.image上创建Graphics,再进行作图。

注意，因pictureBox1.image是最初bitmap创建，默认是黑色，故，须用fillrectangle把背景刷成白色。

Imports System.Drawing
Imports System.Drawing.Drawing2D
Public Class Form1

    Private Sub Button1_Click(sender As Object,e As EventArgs) Handles Button1.Click
        Dim gr As Graphics
        Dim bp As New Bitmap(PictureBox1.Width,PictureBox1.Height)

        Dim pen1 As New Pen(Color.Red)
        pen1.DashStyle = DashStyle.DashDotDot
        pen1.Width = 2

        PictureBox1.Image = bp

        gr = Graphics.FromImage(PictureBox1.Image)
        gr.FillRectangle(Brushes.White,New Rectangle(0,PictureBox1.Width,PictureBox1.Height))
        gr.DrawLine(pen1,10,70,70)

        PictureBox1.Image.Save("D:\1.bmp",System.Drawing.Imaging.ImageFormat.Bmp)

        gr.Dispose()
    End Sub
End Class