用Image画边框怎么画不出来？见内容。

在Form1上放置一个TImage控件，   
Image1->Top    = 100;
    Image1->Left   = 100;
    Image1->Width  = 500;
    Image1->Height = 400;
    Image1->Transparent = true;//透明

    Graphics::TBitmap * pBMP;
    pBMP = new Graphics::TBitmap();
    pBMP->Width  = 500;
    pBMP->Height = 400;
    pBMP->Canvas->Brush->Color = clGreen;
    pBMP->Canvas->Brush->Style = bsSolid;
    pBMP->Canvas->Pen->Width = 1;
    pBMP->Canvas->Pen->Color = clRed;  
    pBMP->Canvas->Pen->Mode  = pmCopy;
    pBMP->Canvas->Rectangle(0, 0, 500, 400);
   
    Image1->Canvas->CopyMode = cmMergeCopy; // 将画布上的图像与源位图进行AND操作
    Image1->Canvas->StretchDraw(TRect(0, 0,500,400), pBMP);
只能显示出一个clGreen的方块，不能显示边界clRed边框。不知为何？