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;