var txt = new Array();
txt[0]="./images/index_header02.jpg";
txt[1]="./images/index_header01.jpg";
txt[2]="./images/index_header03.jpg";
txt[3]="./images/index_header04.jpg";

mmax = 4; //メッセージ行数
txtno = Math.floor(Math.random() * mmax);
document.write("<img src='"+txt[txtno]+"'>");