<?php

// Add to your theme's functions.php
function fwp_image_fix() {
?>
<script>
(function($) {
  $(document).on('facetwp-loaded', function() {
    $('ul.products img').wrap('<div class="thumb" />');
  });
})(jQuery);
</script>
<?php
}
add_action( 'wp_head', 'fwp_image_fix', 50 );