In function wordpress_import_process_post_images() there is this line:

         $newurl = '/'.$images_dir.'/'.$imginfo['basename'];

This will only work if the Drupal site the WP posts are being imported to is at the document root for that URL -- e.g. it is http://example.com rather than http://example.com/subdir/.

Suggested substitution:

$newurl = url($images_dir . '/' . $imginfo['basename'], array('absolute' => TRUE));