This is a hard one but I think this will work. Someone please correct me if I'm wrong.
This uses the external Simple Pie library built into WordPress to fetch the feed, get the image url and create a new post for each item and save the image url as a custom field.
To activate the process we have to hook into wp_cron. The code below does it daily but it would probably be better to do it weekly to prevent overlap. Some overlap will probably occur so this still needs a way to check if we have already imported the image
First we need a function to save the custom field after the post has been created. This section comes from another answer I found on WordPress Answers.
Edit:
This needs to be wrapped in a plugin to schedule the cron event and the cron event was missing the action to make it fire.
Edit: #2:
I Tested the plugin and made a few edits and it fully works except that the RSS feed we are fetching contains relative urls so in the code the slider code you will need to echo out the domain name then the custom field: ie:
'<img src="http://animelon.com<?php get_post_meta($post->ID, 'feed_image_url', true); ?>" />
<?php
/*
Plugin Name: Fetch The Feed Image
Version: 0.1
Plugin URI: http://c3mdigital.com
Description: Sample plugin code to fetch feed image from rss and save it in a post
Author: Chris Olbekson
Author URI: http://c3mdigital.com
License: Unlicense For more information, please refer to <http://unlicense.org/>
*/
//Register the cron event on plugin activation and remove it on deactivation
register_activation_hook(__FILE__, 'c3m_activation_hook');
register_deactivation_hook(__FILE__, 'c3m_deactivation_hook');
add_action( 'c3m_scheduled_event', 'create_rss_feed_image_post');
function c3m_activation_hook() {
wp_schedule_event(time(), 'weekly', 'c3m_scheduled_event');
}
function c3m_deactivation_hook() {
wp_clear_scheduled_hook('c3m_scheduled_event');
}
function create_rss_feed_image_post() {
if(function_exists('fetch_feed')) {
include_once(ABSPATH . WPINC . '/feed.php'); // include the required file
$feed = fetch_feed('http://animelon.com/booru/rss/images'); // specify the source feed
}
foreach ($feed->get_items() as $item) :
// global $user_ID;
$new_post = array(
'post_title' => $item->get_title(),
'post_status' => 'published',
'post_date' => date('Y-m-d H:i:s'),
//'post_author' => $user_ID,
'post_type' => 'post',
'post_category' => array(0)
);
$post_id = wp_insert_post($new_post);
if ($enclosure = $item->get_enclosure() )
update_post_meta( $post_id, 'feed_image_url', $enclosure->get_link() );
endforeach;
}
Screenshot of the new posts pulled in:

Screenshot of the custom field created:
