unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls;

type
  TForm1 = class(TForm)
    Image1: TImage;
    procedure Image1Click(Sender: TObject);
  private
    FPoly: array of TPoint;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Image1Click(Sender: TObject);
var
  P: TPoint;
begin
  P := Image1.ScreenToClient(Mouse.CursorPos);
  SetLength(FPoly, Length(FPoly) + 1);
  FPoly[High(FPoly)] := P;

  Image1.Canvas.Polygon(FPoly);
end;

end.