<?php
$dir = new DirectoryIterator(Mage::getBaseDir('media') . DS .  $sub_dir . DS . $category . DS);
$extensions = "jpg|jpeg|png";
$images = array();

foreach ($dir as $fileinfo) {
    if ($fileinfo->isFile() && stristr($extensions, $fileinfo->getExtension())) {
        array_push($images, substr($fileinfo->getPathname(),strlen($_SERVER['DOCUMENT_ROOT'])));
    }               
}

return $images;
?>