gistfile1.php

<?php
function replaceSrc($matches) {
    global $page;
    
    $url = $matches[1];
    $img = 'images/'.basename($url);

    $handle = fopen($img, 'w+');
    $data = file_get_contents($url);
    
    fwrite($handle, $data);
    fclose($handle);
    
    $page->article_images->add($img);
    $page->save();
    
    $newURL = $page->article_images->last()->url;
    
    $return = str_replace($url, $newURL, $matches[0]);

    return $return;
}

foreach(wire('pages')->get('/articles/')->children() as $page) {
    $page->article_content = preg_replace_callback('/<img.*src="(.*)".*\/?>/U', "replaceSrc", $page->article_content);
    $page->save();
}
?>