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