<?php

# (c) http://anggit.com

$folder = 'images/'; // folder gambar
$images = glob($folder.'*.jpg'); // array
$rand = mt_rand(0, count($images) - 1); // acak
echo '<img src="'.$images[$rand].'" alt="'.basename($images[$rand]).'" />';

# Hasil: <img src="images/xxx.jpg" alt="xxx.jpg" />

?>