tag_to_thumb.php

<?php

$posts_query = new WP_Query(['posts_per_page' => -1]);

$images_query = new WP_Query([
    'post_type'      => 'attachment',
    'post_mime_type' => 'image',
    'post_status'    => 'inherit',
    'posts_per_page' => -1,
]);

$images = $images_query->posts;

while ($posts_query->have_posts()) {
    $posts_query->the_post();
    $tags = get_the_tags();

    if ($tags == false) { continue; }

    foreach ($tags as $tag) {
        $path = parse_url($tag->name, PHP_URL_PATH);
        $matches = array_filter($images, function ($image) use ($path) {
            return strpos($path, parse_url($image->guid, PHP_URL_PATH)) !== false;
        });
        if (count($matches) < 1) { continue; }
        $match = array_shift($matches);
        set_post_thumbnail(get_the_ID(), $match->ID);
    }
}