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Use standard method of Form component (Brush method). This method has Bitmap method, which allows you to fill background of the form by images.

procedure TForm1.FormCreate(Sender: TObject);
var
  MyBitmap: TBitmap;
begin
  MyBitmap:=TBitmap.Create;
  MyBitmap.LoadFromFile('factory.bmp');
  Form1.Brush.Bitmap:=MyBitmap;
end;