[delphi.com.ar]

Puedes hacerlo con el mismo BitBtn:

uses
  JPeg;

procedure TForm1.FormCreate(Sender: TObject);
var
  Jpeg: TJPEGImage;
begin
  Jpeg := TJPEGImage.Create;
  try
    JPeg.LoadFromFile('C:\Mis documentos\baggrund.jpg');
    BitBtn1.Glyph.Height := Jpeg.Height;
    BitBtn1.Glyph.Width := Jpeg.Width;
    BitBtn1.Glyph.Canvas.Draw(1, 1, JPeg);
  finally
    Jpeg.Free;
  end;
end;

Saludos!