When the user clicks on the map, the program adds a new Rectangle to the list, placing it at the location the user clicked.

The map's Paint event handler loops through the markers drawing them.

Finally the Clear Markers button empties the list of markers.

Private m_Markers As New List(Of Rectangle)

' Add a marker.
Private Sub picMap_MouseClick(ByVal sender As Object, ByVal _
    e As System.Windows.Forms.MouseEventArgs) Handles _
    picMap.MouseClick
    Const WID As Integer = 7
    m_Markers.Add(New Rectangle(e.X - WID \ 2, e.Y - WID \ _
        2, WID, WID))

    ' Redraw.
    picMap.Invalidate()
End Sub

' Draw the markers.
Private Sub picMap_Paint(ByVal sender As Object, ByVal e As _
    System.Windows.Forms.PaintEventArgs) Handles _
    picMap.Paint
    For Each rect As Rectangle In m_Markers
        e.Graphics.FillEllipse(Brushes.Red, rect)
        e.Graphics.DrawEllipse(Pens.Black, rect)
    Next rect
End Sub

' Clear the markers.
Private Sub btnClearMarkers_Click(ByVal sender As _
    System.Object, ByVal e As System.EventArgs) Handles _
    btnClearMarkers.Click
    m_Markers.Clear()

    ' Redraw.
    picMap.Invalidate()
End Sub