[ticket/12728] Enforce box model sizing on image attachment thumbnails

PHPBB3-12728
[ticket/12728] Adjust .postimage sizing to 100%

PHPBB3-12728
Merge pull request #2609 from VSEphpbb/ticket/12728

[ticket/12728] Enforce box model sizing on image attachment thumbnails

* VSEphpbb/ticket/12728:
  [ticket/12728] Adjust .postimage sizing to 100%
  [ticket/12728] Enforce box model sizing on image attachment thumbnails