<?php
$images = Array();
$dir = opendir("images/");
while(($image = readdir($dir)) !== false)
{
    if(($image != ".") && ($image != ".."))
    {
        $images[] = $image;
    }
}
closedir($dir);
$image = $images[mt_rand(0, count($images) - 1)];
echo '<p><img src="'.$image.'" alt="image" /></p>';
?>