var
  S, D : TRect;    // S = Source, D = Destination
begin
  S.TopLeft := Point(50,50);
  S.BottomRight := Point(400,400);
  D.TopLeft := Point(0,0);
  D.BottomRight := Point(350,350);
  MyPic.Canvas.CopyRect(D,MyPic.Canvas,S);
  MyPic.Width := 350;
  MyPic.Height := 350;
end;