The script TimThumb is used to display thumbnails (cropping, scale pictures...)
When the picture is called by TimThumb, you have to use the absolute path (example: /wp-content/images/your_picture.jpg)
BUT, Wordpress-popular-posts call the picture with http://www.domain.com, which made TimThumb not showing any picture.
To resolve that, I replaced
$img[1]
by
parse_url($img[1], PHP_URL_PATH)
Works fine now!