posts with thumbnails for 1 category in random order

Hello there. My apologies if there’s a similar post to mine.
I am trying to have only one category displayed with posts with thumbnails BUT I want them showing in random order.
I have managed to get the first part right. I have the posts in one selected category showing in thumbnails.
But I don’t know how to do the random bit. I am very new to this whole malarkey!

<?php
$tab['cat']=15;
$tab['showposts']=15;
query_posts($tab);
unset($tab);

if(have_posts()==TRUE) {
?><ul><?php
while(have_posts()==TRUE) {
the_post();

if(has_post_thumbnail()==TRUE) {
?><li title="<?php the_title() ?>"><a href="<?php the_permalink() ?>"><?php the_post_thumbnail('artistes_gallery'); ?></a></li>

<?php
} }
?></ul><?php
}

wp_reset_query();
?>

Thank you in advance!

You can see it all here: https://www.deschosesadire.net/