//BitBlt - работает
BitBlt(Prntr->Canvas->Handle, 0, 0,Form1->Width, Form1->Height, GetWindowDC(Form1->Handle), 0, 0, SRCCOPY);

//StretchBlt - не работает
StretchBlt(Prntr->Canvas->Handle,0,0,1024,768,GetWindowDC(Form1->Handle),0,0,Form1->ClientWidth,Form1->ClientHeight,SRCCOPY);