StarImage.Parent := self;

が大切。

https://forums.embarcadero.com/thread.jspa?threadID=97048&tstart=0
で教えてもらいました。





procedure TForm3.FormCreate(Sender: TObject);
begin
 StarImage := TImage.Create(Self);
 StarImage.Parent := self;
end;

procedure TForm3.FormClose(Sender: TObject; var Action: TCloseAction);
begin
 StarImage.Free;
end;

procedure TForm3.Button1Click(Sender: TObject);
var
 p1, p2: TPointF;
begin

 p1.Create(20, 2);
 p2.Create(400, 400);

 StarImage.Width := 400;
 StarImage.Height := 400;
 StarImage.Position.X :=40;
 StarImage.Position.Y :=40;
 StarImage.Visible := True;
 StarImage.Enabled := True;
 StarImage.WrapMode := TImageWrapMode.iwFit;

 StarImage.Bitmap.SetSize(400,400);
 StarImage.Bitmap.Canvas.BeginScene;
 StarImage.Bitmap.Canvas.DrawLine(p1, p2, 100);
 StarImage.Bitmap.Canvas.EndScene;
end;