To resolve links' image, the type "metadata" is used, the url is like http://localhost:8080/cms/www/_contents-images/ametys-internal%253Asites/www/ametys-internal%253Aplugins/linkdirectory/fr/ametys%253AdirectoryLinks/http-www-lemonde-fr-rss/_metadata/picture_max50x50/rssb.png?contentVersion=&objectId=directoryLink%3A%2F%2F9a8182dd-082d-4a06-bfab-55c64f33351d
This URL is a url for contents only. Check read access on content is now check for a such url.
So for links, this URL returns a 403 forbidden.