Return raw image proxy for wordpress plugin

I am trying to add a file to my wordpress plugin to return an image from a remote API using an authentication token stored in the plugin's configuration.

This is my current code:

?php
// Make sure we don't expose any info if called directly.
if (! function_exists('add_action')) {
    echo 'Hi there!  I\'m just a plugin, not much I can do when called directly.';
    exit;
}


function hinews_get_image($id, $mimetype) {
    $options = get_option('homeinfo_news_options');
    $parm_token = '?access_token=' . $options['token'];
    $base_url = 'https://myurl.com/';
    $image_url = $base_url . $id . $parm_token;
    header('Content-type: ' . $mimetype);
    $image = file_get_contents($articles_url);

    if ($image === FALSE) {
        return 'Could not retrieve image.';
    }

    return $image;
}


hinews_get_image($_GET['id'], $_GET['mimetype']);
?

This file, currently called images.php in my plugins folder, when referenced directly via URL will of course return the bail-out message, since it is not called from within a wordpress context. I could not find any information on how to invoke this file correctly from within a shortcode-rendered page.

The invoking code is:

?php
/**
* Plugin Name: HOMEINFO News
* Plugin URI: https://www.homeinfo.de/
* Description: News articles provided by HOMEINFO.
* Version: 0.0.1
**/

// Make sure we don't expose any info if called directly.
if (! function_exists('add_action')) {
    echo 'Hi there!  I\'m just a plugin, not much I can do when called directly.';
    exit;
}


include("settings.php");


add_shortcode('hinews', 'hinews_shortcode');


function hinews_shortcode(){
    wp_enqueue_style('hinews.css', plugins_url('hinews.css', __FILE__));
    wp_enqueue_script('hinews.js', plugins_url('hinews.js', __FILE__));
    $options = get_option('homeinfo_news_options');
    wp_localize_script('hinews.js', 'php_vars', $options);
    $parm_token = '?access_token=' . $options['token'];
    $base_url = 'https://myurl.com';
    $articles_url = $base_url . $parm_token;
    $response = file_get_contents($articles_url);

    if ($response === FALSE) {
        return 'Could not load data from API. Check your credentials.';
    }

    $news_list = json_decode($response);
    $result = '';

    foreach ($news_list as $news) {
        $result .= 'h2' . $news-title . '/h2';
        $result .= 'p' . $news-text . '/p';
        $result .= 'br/';

        foreach ($news-images as $image) {
            $php_file = 'images.php';
            $args = array('id' = $image-id, 'mimetype' = $image-mimetype);
            $url_args = '?' . http_build_query($args);
            $image_url = plugins_url($php_file . $url_args, __FILE__);      # What to do here?
            $result .= 'img src="' . $image_url . '" alt="' . $image-source . '"';
            $result .= 'br/';
        }
    }

    return $result;
}
?