show random image in loop if image is not set

My solution was really simple:

$input = array("defaultImage1.jpeg", "defaultImage2.jpeg", "defaultImage3.jpeg", "defaultImage4.jpeg", "defaultImage5.jpeg", "defaultImage6.jpeg");
$rand_keys = array_rand($input, 2);
echo '<img class="story-carousel" src="'.$upload_dir['url']."https://wordpress.stackexchange.com/".$input[$rand_keys[0]].'" alt="">';