Hello Martijn Geerts, 
I just change the text format option in details section of the field that i added and in template i wrote 
<?php
foreach($page->banners as $image) {
            
            $thumbnail = $image->size(350,200);
   echo "<a href='{$image->url}' title='{$thumbnail->description}'><img src='{$thumbnail->url}' alt='{$thumbnail->description}' ></a>";
        }
?>
it started working. 
Thanks