var image = new Array(6)
image[0] = '1.jpg';
image[1] = '2.jpg';
image[2] = '3.jpg';
image[3] = '4.jpg';
image[4] = '5.jpg';
image[5] = '6.jpg';


var min_num = 0; 
var max_num = 5; 
var diff = max_num-min_num+1 ; 
var rnd_number=Math.floor(Math.random()*diff + min_num); 

showimage = "<img src=images/" + image[rnd_number] + ">"


document.write(showimage);