My question really is easy: Can you really import many images at occasionally publish (like the gallery does) but in the <ul> for example ? The gallery import is, I believe, the only method to import several images in the publish, however i can't handle the HTML within this situation. I'm able to be wrong, however i did not find anything about this. Appreciate your help.

You are able to place your images within the gallery without posting it to your publish. Then, within the single publish template, you are able to query the pictures connected to the publish to show them.

Accessories are simply posts mounted on a publish, so let’s use get_posts():

function get_post_images($post) {
    $args = array(
        'post_type' => 'attachment', // Images attached to the post
        'numberposts' => -1, // Get all attachments
        'post_status' => null, // I don’t care about status for gallery images
        'post_parent' => $post->ID, // The parent post
        'exclude' => get_post_thumbnail_id($post->ID), // Exclude the thumbnail if you want it on your article list but not inside an article
        'post_mime_type' => 'image', // The attachment type
        'order' => 'ASC',
        'orderby' => 'menu_order ID', // Order by menu_order then by ID
    );
    return get_posts($args);
}

I would recommend to put this function within the functions.php file.

Inside your single.php file:

<ul>
<?php
    $images = get_post_images($post);
    foreach ($images as $image):
?>
<li><?php echo wp_get_attachment_image($image->ID, 'medium'); ?></li>
<?php endforeach; ?>
</ul>

Wordpress Codex links: