Someone

<?php

$images = array();

$dir = opendir('.');

while (($file = readdir($dir)) !== false) {

	if (is_file($file)) {

		if (strtolower(substr($file, -7)) == '.jpg') {

			$images[] = basename($file, '.jpg');

		}

	}

}

$rand = rand(0,sizeof($images)-1);
echo '<img src="'.$images[$rand].'">';

?>