uses [...] jpeg;

[...]

var
  b1,b2:tbitmap;
  j:tjpegimage;
begin
  b1:=tbitmap.Create;
  b1.width:=fensterbreite;
  b1.height:=fensterbreite;
  b1.canvas.handle:=GetWindowDC(Handle des Fensters, normalerweise form1.handle); // Desktop erfassen
  b2:=tbitmap.create;
  b2.width:=b1.width;
  b2.height:=b1.height;
  b2.Canvas.CopyRect(screen.desktoprect,b.Canvas,screen.desktoprect);
  j:=tjpegimage.create;
  j.assign(b2);
  j.savetofile('C:\Screen.jpg');