show random image in loop if image is not set

I want to show a random image from a few options in my loop if a custom field hasn’t a certain value, or it could be if it had no thumbnail. In any case, it should spit out one of the random images. Instead it’s giving me all of them for each post that satisfies the condition. How can I make it show just one at random? Thanks. Here’s the code partial:

<?php

    $loop = new WP_Query( $args );
    while ( $loop->have_posts() ) : $loop->the_post();
    ?>

    <?php $image = get_post_meta($post->ID, 'myimage', true); ?>

    <?php if ($image != 'myimage.jpeg'){
        $upload_dir = wp_upload_dir();
        echo '<img class="myclass" src="'.$upload_dir['url'].'/'.$image.'" alt="">';
    } else { 
    ?>

    <?php 
    for ($i = 1; $i < 4; $i++) { ?>
    <?php echo 'defaultImage'.$i .'.jpeg';

    } ?>

    <?php

    $settings = get_option( "defaultImage" ); 
    $random = rand( 1, 4 );

    if ( isset( $settings[ "defaultImg$random" ] ) )
    echo $settings[ "defaultImg$random" ]; 

    }
?>

SOLVED:
My solution was really simple:

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