var pic = new Array() 

pic[0] = 'pic1'
pic[1] = 'pic2'
pic[2] = 'pic3'

var a = 0
var b = pic.length;

var preBuffer1 = new Array()
for (i = 0; i < b; i++){
   preBuffer1[i] = new Image()
   preBuffer1[i].src = pic[i]
}

var whichpicImage = Math.round(Math.random()*(b-1));
function showpicImage(){
document.write('<img src="images/'+pic[whichpicImage]+'.jpg" alt=" "/>');
}