You’re going to need to alter the field so that it returns the attachment ID instead of the URL. For more information see WP doc for the functions I use here.
<?php
$images = get_field('galerie_bd');
if( $images ): ?>
<ul class="rslides">
<?php foreach( $images as $image ): ?>
<li>
<a href="<?php echo get_attachment_link($image); ?>">
<?php
$size = 'thumbnail';
$attach = wp_get_attachment_image_src($image, $size);
?>
<img src="<?php echo $attach[0]; ? />" />
</a>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
