In system.yaml, twig compatibility must be true

[paulrudy]

Just a comment for future users who may run into issues as I did:

In system.yaml, if you have a strict_mode section, the following results for me in the author image's HTML being rendered as raw text on the page (<img src=... </img>) instead of an image:

strict_mode:
  twig_compat: false

Setting twig_compat: true fixed the issue.