Código PHP:
<?php
$dir = "/images/headerimages";
$dh = opendir($dir);
while (false !== ($filename = readdir($dh))) {
$files[] = $filename;
}
//$files = scandir($dir);
$rnd = rand(2, count($files)-1);
$imgname = $files[$rnd];
?>
<img src="/images/headerimages/<?php echo $imgname;?>" />