var txt = new Array();
txt[0]="./img/cover001.jpg";
txt[1]="./img/cover002.jpg";
txt[2]="./img/cover003.jpg";
txt[3]="./img/cover004.jpg";

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