importing data from non-wordpress mysql db

I have created an import file to get the articles from a non-wordpress db and bring them in as posts in wordpress, I am not sure how to bring over the thumbnail image into my custom field I defined as “thumbnail”. Any suggestions.

Here is the code I am using:

    $results = mysql_query("SELECT headline, abstract, article, author, posted,    category, img, facilities FROM articles",$lclink);

    $i = 0;
while ($row = mysql_fetch_array($results,MYSQL_ASSOC)) {
   $post = array();
   $post['post_status'] = 'publish';
   $post['post_category'] = array(1);
   $post['post_date'] = date('Y-m-d H:i:s',strtotime($row['posted']));
   $post['post_title'] = $row['headline'];
   $post['post_excerpt'] = $row['abstract'];
   $post['post_content'] = $row['article'];
   $post['post_author'] = $row['author'];



   $posts[$i] = $post;
   $i++;
}

require('./wp-load.php');

foreach ($posts as $post) {
  wp_insert_post($post);
}