var spifs = new Array()
spifs[0] = '<img src="img/main-back99.gif" border=0 width=292 height=276>';
spifs[1] = '<img src="img/train06.gif" border=0 width=292 height=275>';
spifs[2] = '<img src="img/back-main999.gif" border=0 width=292 height=276>';
spifs[3] = '<img src="img/main-back9999.gif" border=0 width=292 height=276>';

var max = 4
var randomNum = Math.random(); 
randomNum = randomNum * max;
randomNum = parseInt(randomNum);
if(isNaN(randomNum)) randomNum = 0; // for Netscape
document.write(spifs[randomNum]);