random portfolio post sorting

Hi everyone,

I’m a novice here and couldn’t find a solution to my issue.
I’m using “The 7” wordpress theme from Dream Theme and my index page is a portfolio, not a blog. Currently, this portfolio consists from book covers.

I need to make these covers appear in random order every time the user refreshes the page. How do I do it?

P.S. There is no built-in functionality in this theme and I tried things like inserting this code ‘<?php query_posts($query_string . ‘&orderby=rand’) ?>’ in index.php, but it didn’t help.