banner = new Array(6);
banner[0] = 'all1.jpg' 
banner[1] = 'all2.gif'
banner[2] = 'all3.gif'
banner[3] = 'all4.gif'
banner[4] = 'all5.jpg'
banner[5] = 'all6.gif'

index = Math.floor(Math.random() * banner.length);
document.write('<IMG SRC=images/'+banner[index]+' border=0 width=640 height=140>');