<?php
$images = array(
'image-01.png',
'image-02.png',
'image-03.png',
'image-04.png',
'image-05.png',
);
$image = $images[array_rand($images)];
$output = "<img src=\"http://domain.tld/path/to/image/directory/" . $image . "\" alt=\"Refresh browser for random image\" />";
echo $output;
?>