var txt = new Array();
txt[0]="index_photo/1.jpg";
txt[1]="index_photo/2.jpg";
txt[2]="index_photo/3.jpg";
txt[3]="index_photo/4.jpg";
txt[4]="index_photo/5.jpg";
txt[5]="index_photo/6.jpg";

mmax = 6; //メッセージ行数
txtno = Math.floor(Math.random() * mmax);
document.write("<img src='"+txt[txtno]+"'>");