Public Class Form1

    Dim X1 As Single
    Dim X2 As Single
    Dim Y1 As Single
    Dim Y2 As Single
    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
        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(RedPen, X1, Y1, X2, Y2)

    End Sub

End Class