Set images within posts as their thumbnails

migrate-photos-to-post-thumbnails.php

<?php

$thumbnail_width = 593;
$thumbnail_height = 400;
foreach(query_posts('posts_per_page=-1') as $post)
    if(! has_post_thumbnail($post->ID))
        foreach(get_children('post_type=attachment&post_mime_type=image&post_parent=' . $post->ID) as $image){
            $sizes = wp_get_attachment_metadata($image->ID);
            if($sizes['height'] >= $thumbnail_height && $sizes['width'] >= $thumbnail_width){
                set_post_thumbnail($post->ID, $image->ID);
                break;
            }
        }