Drupal's default oEmbed template is extremely sparse, and some accessibility checkers don't like that.
Drupal's core “Manage display” functionality provides the option to link images to content, which is handy for, e.g., teasers. If the link contains nothing but an image, however, some accessibility checkers will (rightly) flag these links because there's no textual information indicating where they're going. One solution is to add a
title attribute to the link.