simg=new Array(4);
simg[0]="index_img/img_subB.jpg";
simg[1]="index_img/img_subB1.jpg";
simg[2]="index_img/img_subB2.jpg";
simg[3]="index_img/img_subB3.jpg";
Myimg=Math.floor(4*Math.random());
document.write("<center><img src='"+simg[Myimg]+"'></center>");