function random_image(){
  var text_st = new Array("head_pic_01", "head_pic_02",
  "head_pic_03", "head_pic_04", "head_pic_05");

  var l = text_st.length;

  var rnd_no = Math.round((l-1)*Math.random());

  //alert(text_st[rnd_no]);
  var image = "<img src=\"/images/" + text_st[rnd_no] + ".jpg\"/>";

  document.write(image);
}