Use the PHP shuffle
function to randomise the array:
$out .= '" id="_'.$menu_id.'" style="background-color:'.$background_color.'">'."\n";
$out .= ' <div class="shadow"></div>'."\n";
$out .= ' <div class="content">'."\n";
$out .= ' <div class="text">'.balanceTags($content).'</div>'."\n";
$out .= ' <div class="marquee-outer">'."\n";
$out .= ' <ul class="marquee-inner">'."\n";
$images_array = explode(',', $images);
shuffle($images_array);
foreach($images_array as $image_id){
$image = wp_get_attachment_image_src($image_id, 'full');
$out .= ' <li class="marquee-item" style="background-image:url('.$image[0].');"></li>';
}
$out .= ' </ul>'."\n";
$out .= ' </div>'."\n";
//$out .= ' </div>'."\n";
//$out .= '</section>'."\n";
return $out;
}