Var
  DrawLine: Boolean;
  Xs, Ys, Xo, Yo : Integer;

 procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
  begin
   IF Button = mbLeft
    then
     Begin
      DrawLine := True;
      Xs := X;
      Ys := Y;
      Xo := X;
      Yo := Y;
     End
    else ;
  end;

 procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
  begin
   IF Button = mbLeft
    then
     Begin
      DrawLine := False;
      Image1.Canvas.MoveTo( Xs, Ys );
      Image1.Canvas.LineTo( Xo, Yo );
     End
    else ;
  end;

 procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
  begin
   IF DrawLine
    then
     Begin
      Image1.Canvas.Pen.Mode := pmNot;
      Image1.Canvas.MoveTo( Xs, Ys );
      Image1.Canvas.LineTo( Xo, Yo );
      Image1.Canvas.MoveTo( Xs, Ys );
      Image1.Canvas.LineTo( X, Y );
      Xo := X;
      Yo := Y;
     End
    else ;
  end;