The line its painting the picturebox - vb.net

Question

I'm doing a program to make a polygon and fill the outside of it, all of this on an image of a picturebox.
My code is this (I extracted a part of the original and I used and edited the code of VB Helper: Let the user draw polygons in Visual Basic .NET)
Dim Polygons As New List(Of List(Of Point))()
Dim NewPolygon As List(Of Point) = Nothing
Dim NewPoint As Point
Dim bmp As Bitmap
Private Sub khe_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
bmp = New Bitmap(PictureBox1.Width, PictureBox1.Height)
PictureBox1.Image = bmp
End Sub
Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
If (NewPolygon IsNot Nothing) Then
If (e.Button = MouseButtons.Right) Then
If (NewPolygon.Count > 2) Then _
Polygons.Add(NewPolygon)
NewPolygon = Nothing
Else
If (NewPolygon(NewPolygon.Count - 1) <> _
e.Location) Then
NewPolygon.Add(e.Location)
End If
End If
Else
NewPolygon = New List(Of Point)()
NewPoint = e.Location
NewPolygon.Add(e.Location)
End If
PictureBox1.Invalidate()
End Sub
Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove
NewPoint = e.Location
PictureBox1.Invalidate()
End Sub
Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
Using myGraphics As Graphics = Graphics.FromImage(PictureBox1.Image)
For Each polygon As List(Of Point) In Polygons
Dim gp As New GraphicsPath
gp.AddPolygon(polygon.ToArray())
Dim rgn As New Region(gp)
myGraphics.ExcludeClip(rgn)
myGraphics.FillRectangle(Brushes.LightPink, Me.ClientRectangle)
myGraphics.DrawPolygon(Pens.SeaShell, polygon.ToArray())
Next polygon
If (NewPolygon IsNot Nothing) Then
If (NewPolygon.Count > 1) Then
myGraphics.DrawLines(Pens.SeaShell, _
NewPolygon.ToArray())
End If
If (NewPolygon.Count > 0) Then
Using dashed_pen As New Pen(Color.Aquamarine, 5)
myGraphics.DrawLine(dashed_pen, _
NewPolygon(NewPolygon.Count - 1), _
NewPoint)
End Using
End If
End If
End Using
End Sub
The problem is this: Initially I was using e.Graphics and the program was working well, but this does not paint the image, rigth? and I changed it, but when I click to start drawing and I move the mouse to set the next point the line is painting the image wherever I move, like this.
This is bad because I want to get a image like this one, and with this problem if I move the mouse on the area I want to keep its going to be painted and the image will be useless.
I'm using Visual Studio 2008 and I'm sorry if my english it's not the best.