Public Class Form1

    Dim x1 As Integer
    Dim X2 As Integer
    Dim Y1 As Integer
    Dim Y2 As Integer
    Dim RedPen As New Pen(Color.Red, 3)

    Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown

        If e.Button = Windows.Forms.MouseButtons.Left Then
            X1 = e.X
            Y1 = e.Y
        ElseIf e.Button = Windows.Forms.MouseButtons.Right Then
            X2 = e.X
            Y2 = e.Y
            PictureBox1.Refresh()
        End If

    End Sub

    Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
        e.Graphics.DrawLine(Pens.Black, x1, Y1, X2, Y2)
    End Sub

End Class